It seems to me that if you want to offer something for sale, the auction is the way to go.

On the other hand of you are looking for something like an instrument to buy, putting up a thread about it makes sense, and that means inviting advice about where to find it, including saying they might have something suitable themselves.

There are times when sticking the offer in a thread does make sense, for example when someone has tickets for a festval, or a room booked, and they've had to change their plans, and it might just solve someone else's problem.