Exactly. FABO doesn't control who you sell to. How long you wait on receiving payment is entirely up to the seller. If people's feelings get hurt because they contacted you first but then disappeared for days without paying and someone else had the money, well, that's their problem. Saying "I'll take it" is not an indefinite hold.
If it's causing you stress, just do as S'cuder says and put the terms in the sale ad. 24 hours is pretty short, so, putting it in the ad will help explain your level of comfort with that type of thing. I understand where you're coming from, I wouldn't wait a week without re-selling it, but I have waited a few days because not everyone on this board is all that internet savvy. But that timeline is 100% in your hands as the seller.