I only put stuff on hold if the person is absolutely sure that they will buy, and can give me a date when they will pay (if it's someone I trust, a couple weeks is no problem). If they stop communicating/don't pay in the time agreed, I would feel free to leave them neutral feedback tbh. Sick of time wasters!
If someone just asked you to put something on hold for them but didn't specify a time frame, I would put it back up for sale after two days or so if they didn't communicate.