First dibs / my word is my bond

On some of the fora I am on, there is a rule in the Sales section which stipulates adverts can only be placed on the understanding that the first offer to buy (by posting on the thread) gets the goods. This avoids any hassle about when a pm was sent/received, sold via SMS etc.

If a seller fails to follow the rules they are named/shamed and banned from all future sales activity. Sounds good to me.