I'm with Ghost and Ebby on this one.
A deal should be a deal. If a buyer clearly says they'll take it, that should be it. No problem for a seller to let other possible buyers know they'd be next in line if the deal falls through for some reason. And, if a buyer backs out of something that was meant to be definite, then they can be named in the good guy/bad guy list.
I've got to say, though, that in my experience of selling online on forums where people value their own reputation and honor, I've rarely had a buyer not complete a sale.
What I have seen here, though, is sellers not giving complete details (or photos!) so that buyers can make an informed decision. Or, worse, state that something is complete when it's not. I've contacted the seller, told them I'd like the item, but asked if it's complete ... only to have it sell to someone else. If an item is under negotiation partly because the seller hasn't been thorough, the seller should provide all the relevant info to the person who has expressed interest so the buyer can make the call.
I may disagree here.
Buyer A has question or needs another photo or some other contingency
Buyer B says I'll take it. What's your PayPal ?
Buyer B gets it.
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