From a seller's standpoint, it can be tough to sell something out from under someone who has said they'll buy it but hasn't had the time to send the funds yet. I've had this happen with time zone differences between me and the buyer. Or if they ask a question (e.g., "Will it come shave-ready? If so, I'll take it," for a straight razor) but can't read my reply until they get off work or some such. There can be a time lag in communication and I personally would feel bad if I were to sell it out from under the person in such situations. No moral condemnation if a different seller made a different choice than I do, but this is why I have put "sale pending" on different forums in the past.
Also possible is lags due to different methods of payment: bank transfers, checks, etc. Though I suspect those are much less common.