By that point it's a face-to-face transaction, which is arguably easier to judge, and the buyer has had a chance to look at the bike. If it's really that bad, it'll be obvious during the inspection - do the wheels seem true, the bearings seem smooth, the headset seem right, and everything seem adjusted? It's not completely free of danger - you could find that the bike seemed OK but isn't, or that it's just not what you want, and have the seller refuse to buy it back despite the promise. You could also have the seller mug you, take your money, and ride off on the bike, whether or not there's a buyback promise on the table. Any transaction requires some element of risk and/or trust.