If you have taken a bag of unwanted items to a charity shop recently, there is a good chance one of the volunteers will have tapped you on the shoulder and asked you to sign up to a gift aid scheme.

A growing number of charity shops are inviting people to boost the value of their donated clothes, books and CDs simply by filling in a gift aid declaration­ form. This allows the charity­ to reclaim gift aid on the amount of money that your unwanted jumper or pair of shoes is sold for.

In many ways it's a no-brainer. Signing­ the form means you are helping­ to raise vital extra funds "at no additional cost to you," as the British Heart Foundation puts it. If your bag of books sells for £30, the charity can reclaim an additional £8.46.

But Guardian Money can reveal that the way the gift aid scheme works means it is arguably open to being exploited by less charitable individuals,­ who could end up making money at the expense of good causes.

Meanwhile, HM Revenue & Customs has warned that giving your stuff to charity shops to sell in this way can have tax implications for some individuals (see below).

And you may be surprised to learn that if there is a problem with one of the items you donated – for example, someone who buys one of your DVDs demands a refund because it won't play properly – the official line is that it's your responsibility, not the shop's.

Most Guardian Money readers will probably have come across gift aid as boosting the value of donations of money. But back in 2006, Sue Ryder Care found a way of applying the same principles to donated goods.

The shop offers to act as the donor's "agent," selling his or her items on their behalf.

Typically, when you drop off a bag of stuff, a volunteer asks you to fill in a simple form to join their scheme. You are then usually given a card carrying­ your unique ID number, and the items you donated are tagged with this number (usually a sticker).

When they are sold, the charity contacts­ you to confirm that you consent­ to the proceeds being donated. This then allows­ the charity to claim gift aid on the donation.

This increases the value of donated items by 28%, so a dress sold for £5 would actually raise £6.40. After that, each time you take items to the shop, you simply show your card.

Not surprisingly, the idea has really taken off. Oxfam started rolling out its "Tag your Bag" gift aid scheme in February last year, and, so far, about 580 of its 700 shops have been signed up, with the rest likely to be fully on board within weeks.

Sue Ryder Care, British Heart Foundation, Help the Aged and PDSA are among the other big-names that run similar initiatives at their retail outlets, and the disability charity Scope is triallin­g it at 16 shops from next month ahead of an expected national roll-out.

David Moir at the Association of Charity Shops says the scheme helps to build up a good relationship between the organisation and the donor. However, he adds that it is burdened with complexity, and his body is lobbying the government to make the rules simpler.

They are surprisingly onerous – in its small print, Revenue & Customs points out that the donated goods remain the property of the person donating them until they are sold. It must be clear it is the individual who is selling the goods, and not the charity, which is merely acting as an agent.

"The owner must have the right to keep all or part of the proceeds of the sale, but can choose to donate all or part of the proceeds if they wish," says the Revenue, which adds that the charity­ "must contact the potential donor­ after the goods are sold and offer to pay them the proceeds".

The taxman also stresses that the wording of the gift aid form mustn't commit people to making a donation.

In other words, you are legally (if not morally) entirely within your rights to take bags of stuff to your charity shop, let them do the hard work of selling it, and then decide to keep the money.

"Unfortunately, if this happens, there is nothing the charity can do. The goods are the property of the owner until they are sold, and after they are sold, they have the right to keep some or all of the proceeds," says the Institute­ of Fundraising. It adds: "Hopefully this is unlikely to happen, as the person is donating the item to charity and didn't originally expect to gain from it."

David Moir says only a "very, very small percentage" of people will say yes, I'll take the money. He adds that, anecdotally, it appears many of these people misunderstand what they have been asked – for example, they might think they can take the cash and the charity shop gets the gift aid.

In order to make sure they are not left completely out of pocket if someone decided to keep the proceeds, some charities will charge a sales "commission" for the costs of acting as agent. Oxfam's is 1% of the sale price — a surprisingly small amount, some might say.

Revenue & Customs rules also suggest you should check very carefully that any items you are planning to donate are in good nick.

It says a charity must make donors aware that, in the unlikely event someone who buys one of their items returns it and demands their money back, "it will be their responsibility, rather than that of the charity". (That said, one suspects many shop volunteers would ignore this requirement and simply hand the unhappy purchaser a refund).

So, before you take that bag of stuff down to Oxfam, make sure all the pieces of the jigsaw are in the box, those old Ladybird books haven't been scribbled in, and that that VHS copy of Mission Impossible hasn't been taped over with Wimbledon coverage ...

What it says in the small print

Revenue & Customs says the income generated from the sale of donated items "may have tax implications" for the person who gave them to the charity shop.

In the small print of the gift aid scheme rules, it warns that if someone sells "significant quantities" of goods this way – with the shop technically acting as their agent – "they must consider their own position in terms of income tax and VAT on the proceeds, even though they may donate them to the charity" (our italics).

Writing this month in Taxation magazine, Richard Curtis says one point to remember when adopting this method, is that if the charity is selling on the supporter's behalf, "then for tax purposes, it is as if the supporter himself is making the sale".

He adds: "If a chargeable asset, with a substantial value, was sold by a charity under this scheme – say some jewellery or a painting – then a liability to capital gains tax could arise on the charity supporter."

The Revenue says the owner of a valuable item therefore may want to think about giving it directly to the charity instead, as they will not be charged CGT on any chargeable gains on gifts of assets to charities.

However, the charity wouldn't be able to claim gift aid on their donation.