While you may already be purchasing new products that are deemed environmentally friendly, you can actually contribute to a sustainable ecology by buying used items, especially those that take a lot of energy and resources to produce. Of course, if you buy products used, you're likely to save yourself a ton of cash.

There are a few caveats to buying used products: You can get burned if you're not careful. Educate yourself about any product you intend on purchasing and know how to inspect it defects and hazards. Also, learn the value of any used item that still may carry a hefty price tag by checking out similar sales at online resale sites.

Before you head out to the thrift shops, check to see if there are local reuse or freecycle resources in your area. The Housing Works Bookstore Cafe in New York City is a volunteer-run venue that sells used books, CDs, DVDs, and LPs they receive as donations. In the San Francisco Bay Area, the East Bay Depot for Creative Reuse in Oakland promotes solid waste diversion and conservation by selling used art materials, educational supplies, furniture, and home products. You can find such an organization or venue in your area by visiting the Freecycle Network, FreeLocal.org, Resale Shopping and the Thriftshopper.

Here's some products you should consider buying used instead of new.

Bicycles. There is little need to pay full price for a children's bicycle as they are quickly outgrown, and you can find a decent commuter bike for about the price of two tanks of gas. But you have to know where to look before getting a used bike. Be wary of private sales —especially for pricier, adult bikes — as stolen bikes make up a large part of the market.

One great place to look for mountain bikes and hybrids is at bike-rental shops. You'll frequently find deals in the autumn. It also wouldn't hurt to leave your contact information with a bicycle repair shop and ask them to call you if a bike sent for repair was abandoned by its owner.

I observed as an acquaintance sought and purchased a hybrid bike. The Specialized bike he purchased retails for around $500 new, but he found a used one for $100, although it needed a new saddle, tires and brakes. If you don't know how to spot worn items, ask the seller if you can can take the bicycle to a repair shop to have it looked over.

Exercise equipment. Weight-lifting equipment is a staple at yard sales, and you can also find some remarkably inexpensive weights on Craigslist. The quality of weights, especially those that are all metal, isn't likely to deteriorate over time; a good set should be indestructible. You should always check for broken and missing pins, bolts and fasteners. If the weights are rusted, be sure it's something you can clean without compromising their integrity.

You should be much more careful when buying exercise equipment with movable parts, such as treadmills, ellipticals and exercise bikes. Over time, they can flex and become less stable, and electronics are more prone to fail over time. If you're interested in buying an exercise machine, you should literally give it a workout and see how it performs. I recently found a NordicTrack exercise bike that retails for $800 for $100 on Craigslist and it had only moderate wear.

Musical instruments. Used musical instruments can be had at steep discounts. If you're intending to rent or buy a musical instrument for a child who is a beginner, it makes much more sense to buy one used. Renting a trumpet for a year might cost $200, but buying a similar used one in a private-party sale may cost only a little more. If the child doesn't take to the instrument, you can always resell it.

Take any instrument you're interested in buying to a repair shop to determine if any work or maintenance needs to be done before you purchase it. It's more risky buying a used musical instrument online, as you might not be able to have it inspected.

Wooden furniture. You should have no problem finding a lot of good wooden furnishings on the market, especially in areas where apartment dwellers don't want to haul their furniture with them when they move. While you might not pay a penny for some less desirable furniture, you should never expect to pay more than a third of what the furnishings cost new.

New or used, you get what you pay for when it comes to furniture. Furniture made from cheap pine or particle board won't hold up nearly as well as oak, maple, walnut, and cherry. Also look for well-built furniture. Joint construction is the main determinant of quality furniture. Anything constructed with staples or nails, or visible glue indicates cheap construction. Look for dowels, screws, dovetail joints, and reinforcement blocks at corners.

The obvious place to check is Craigslist, but you should also check out estate sales, consignment shops, antique stores, yard sales, or even online freegan groups. (Take care when purchasing upholstered furniture or mattresses that they don't carry bedbugs or other invasive pests. This may be rare, but it pays to be careful.)

Dishware and utensils. Kitchen goods aren't always a good bet to buy used. A mixer might have a worn motor or electrical problems and used blender blades are often dull. However, there's nothing wrong with buying utensils and dishes, especially if you can find them in matching sets.

Thrift stores and estate sales are often the best places to look. Be careful to examine any utensils and dishes for cracks, chips and missing enamel. China and glassware may also have invisible stresses that could become cracks.

Stainless steel flatware is a more attractive option than silverware, especially if the silverware has lost its plate. While the base metal for silverware is often copper or brass, a tin-alloy base may pose health risks. Also, be sure any kitchen product you purchase does not contain chemicals and coatings such as Teflon, melamine resin and Bisphenol-A, which may pose health risks.

Vehicles. From a sustainability perspective, it definitely makes more sense to buy a used car that gets good fuel economy over a new one, as the building and disposal of an automobile has a significant environmental impact. The automotive industry's own studies have shown that between 12 and 28 percent of the carbon dioxide emissions generated during a car's lifecycle occur during the acquisition of raw materials, manufacturing and initial shipment. Every time a consumer opts for a used car over a new one, that's one less vehicle headed to the scrap heap.

As far as value is concerned, the old adage rings true: A car loses value as soon as you drive it off the dealership lot—about 9%. It doesn't stop there; by the time it's four years old, the typical car retains only about half of its value. After that, however, the depreciation curve is much less steep. And at that age, most cars remain reliable for many years to come. Your continuing ownership expenses such as insurance and taxes should be lower as well.

Overall, car dependability has improved considerably over the past decade. A five-year-old vehicle is likely to have about a third fewer problems than one would have had a decade back. Engine, transmission and exhaust problems, which once plagued older cars, have decreased dramatically, according to automotive research groups. When properly maintained, a vehicle built in 2009 should easily last another six to eight years without needing any major repairs. Modern cars can last 150,000 miles or even much longer when properly maintained.

The downside to buying a used car is that it is typically out of warranty, and you'll have to pay for repairs yourself. However, most will be for wear items such as brakes, batteries and tires. While there's always the risk you'll get a car that was poorly maintained or a lemon, you can mitigate those concerns by taking any car you're interested in buying to a certified mechanic for a thorough inspection.

Clothing. If it grosses you out to buy used clothes, consider this: There's a good chance some of the clothes you bought “new” were tried on by several people at the department store before you paid top dollar for them. While it's understandable to draw the line on undergarments, stockings and shoes, there's nothing about used clothes that one good hot-water cycle can't cure.

Buying clothes from thrift stores doesn't limit you to dowdy, out-of-style duds; you can buy the same clothes still on sale at department stores at less than a tenth of the cost, or even less. We recently purchased a pair of Hudson designer jeans, which sell for about $190 new for only $4 at a Goodwill Thrift Shop, and they were notably unworn.

Some clothes just make sense to buy used. Maternity clothes are typically worn only a few months. Babies and children grow so quickly that they're likely to outgrow a pair of hand-me-downs before they're worn out.

Books. E-books may be a greener — and often less expensive — option to buying paper books, but buying those paper books used is even less expensive and probably better for the environment. Used bookstores and online resale sites often have popular titles, although you may have to wait a short time until they reach their shelves. Metasearch engines such as AllBookstores.com will allow you to search multiple retailers to find the lowest price. An even cheaper option is to scour flea markets and yard sales, where you can often find books for less than a dollar. You should also look to see if your area has a book exchange, and ask your local library when it shifts its inventory and how it disposes of its old books.