How about this, when you email to confirm you have a record still available, you say it will be reserved for 24 hours, and then if you haven't received payment it will become available to anyone again. That way, using me as an example, I could email you Monday night asking if you still had a record, you would reply Tuesday morning, but I'm at work and don't get home till 5 pm so don't see your email until then. But when I read it, I can either pay within the 24 hour period, ie when I get home from work, or at least email you to say I'm sending a cheque, or it becomes available to anyone else.
ps have you still got the Ray Pollard ?