MOM-AND-POP retailers have helplessly stood by over the last decade as big-box merchants steamrolled over them. Online, though, small merchants are not going down without a fight.

The number of small- and medium-size retailers selling online has swelled in the last two years, from 21 percent to 32 percent, according to a survey by IDC, a consulting firm. Aided by less expensive and more sophisticated technology, stores like RealmDekor.com, CleanAirGardening.com and SitStay.com are competing with retailers as well as bigger sites like Amazon.

These businesses lack the huge marketing budgets of their bigger peers, of course, but they are unearthing cheap advertising methods that, in some cases, help them compete with million-dollar promotions.

The retailer of quirky home goods, RealmDekor.com, has experienced occasional sales increases not because of catalog shipments or television commercials, but because it formed relationships with bloggers and posted its products on new “social shopping sites” like ThisNext.com and StyleHive.com.