Here are some start-ups that offer new online twists on thrift stores and yard sales:

Copious: A "social marketplace" for buying and selling through your extended social networking contacts.

Threadflip: Offers both do-it-yourself and concierge-type services to let users resell fashion items.

ThredUP: Online marketplace for secondhand children's clothing.

Twice: Buys and photographs customer clothing, then resells on its website.

Poshmark: Users upload photos of their items, then troll virtual "Posh Party" boutiques for upscale bargains.