OfferUp - The Largest Mobile Marketplace: Buy, Sell, Offer Up.

Just like the title states this app lets you make offers on products you are purchasing. The sellers can then look at your offers and accept, decline or counter with another offer. Buyers and sellers are rated on their transactions which then appears in their profile and gives buyers and sellers an idea of who they can trust. This system is similar to Ebay's, but probably a little simpler. I'm not encouraging you to quit your day job, but this should provide some supplemental income if your a seller on OfferUp. It should also help you find some good deals if you're a buyer.  And with the recent addition of a shipping option there's even more deals to be had!  Sign up here and receive $5 off your first purchase.