While it has been some time since I sold anything here, when I did I typically noted time-limits (12/24 hours because I have no patience) for people to respond/pay before I would move on to the next buyer, and "until you say/write 'I'll take' you don't have a claim on the knife'". It probably rubbed some people the wrong way and/or reduced the pool of prospective buyers, but sure reduced my frustration.