If you're on the buying side, you want to make sure the seller doesn't sell that amazing find before you get there. Or if you're on the selling side, you want to make sure the buyer actually shows up after you've told 8 people "sorry it's sold."
In the big world of real estate and cars and such, we call this "escrow". A trusted 3rd party holds *some* of the purchase price, so both parties feel safe and confident to continue the transaction.
In the Craigslist, OfferUp, Facebook, garage sale world, we call it Lockbox Local. It's a simple app that lets buyers make a deposit and hold their item and sellers get a deposit so they know it's sold.
Listen to see how the pitch for Lockbox Local sounds. Would you use it after hearing this pitch?