It really doesn't matter where and how the deal was done. A deal is a deal - period. We can argue about symantics all day long, but deals get done in the sales thread itself, in PMs and on eBay. People have been added to the PL or HOS in all 3 situations listed above. As soon as the buyer says "OK, I'll take them" then that constitutes a sale and both buyer and seller need to fulfill their obligation to complete the deal.
I am afraid it is a valid point simply because there is no list of rules posted for a WTB thread in the way there are in a selling thread.
Customs are customs and posted rules are posted rules.
Compromise is a two way street.