With eBay, unless you are "ASKING" a question, everything else is a contractual agreement, so there are consequences for not abiding to the transactions.
Currently, OfferUP is a FREE TO USE app that ONLY faciliates the POTENTIAL transaction between a seller and a buyer...NOTHING MORE. Users on OfferUP rely on the honor system, but they are testng out "listing fees" and alternative payment systems, in which case, they will be emulating eBay down the road.
BTW, having an option to..."accept or decline an offer by tapping a button. If accepted, the app should prompt the buyer to propose 2 or 3 meeting locations and times."...is useless without a contractual agreement, since there are no consequences.