SAN FRANCISCO, United States — The luxury-brand jacket that fits perfectly and has a tiny price tag is the holy grail of the consignment store shopper. Prada, Chanel, Burberry, Gucci—the truly tenacious bargain hunter can find all these brands at the pre-loved price at the right secondhand outlet.

The largest online consignment store in the U.S., however, is stockpiling clothes that were never that costly to begin with: ThredUp says some of its most popular brands are H&M, Forever 21, and Old Navy. In fact, it won't even accept used Tommy Hilfiger clothes because there isn't enough demand from its target customers—25- to 40-year-old women.  H&M and other discount "fast-fashion" labels cost next to nothing by the time they're for sale on ThredUp and customers are adding them to their checkout carts impulsively as they would packs of gum at the supermarket counter.

“I think it has to do with the fact that they’re very trendy styles, and people are probably cycling through them quickly,” says ThredUp spokeswoman Farrell Klein. “You’re seeing things that were very recently in the store.”

Of course, not all e-consignment stores regard fast fashions kindly. Twice, also based in San Francisco, rejects all three: H&M, Forever, 21, and Old Navy. “They’re really great brands, but they tend not to last,” says Chris Luhur, Twice’s director of marketing and community, either because they’re poorly made or too trendy. “Something that was really hot last season people don’t really want to buy anymore,” she says. The company does accept select Tommy Hilfiger items.

But the two resellers do agree on the bestselling category—handbags. Marc Jacobs, Kate Spade, Michael Kors, Coach, and Rebecca Minkoff are consignment gold. For Twice, so are Vera Bradley's quilted patterned bags. "It's an unlikely brand to be popular," says the e-retailer's operations manager, Erica Setness. "But it has one of the highest sell-through rates of all the handbag brands." Twice and ThredUp operate the same way: They receive and sort through the clothing people send to them in designated shipping bags.

The online shops also place a premium on athletic attire from Athleta, Under Armour, and especially Lululemon, whose yoga pants retail for around $100. "Those never last more than a day or two; they literally fly off the site," says Twice's Luhur.

Overall, ThredUp and Twice favor brands that make classic styles solidly constructed from quality fabrics, such as Tory Burch, Lilly Pulitzer, and Boden. "When I started, I was more into more edgy brands," Setness says, who counts Isabel Marant among her favorite fashion designers. "Now I get most excited about J.Crew."

By: Belinda Lanks; editor: Alex Dickinson.