"I'll pay you later today"

I've been buying and selling on Audiogon for a while, and as a seller I've had my share of tirekickers and "What's your best price?" fishermen. Lots of people ask questions, then are never heard from again. That doesn't bother me. The one thing I haven't experienced on Audiogon until recently has been someone who commits to a deal and then doesn't pay.

The last several times I've attempted to sell something, I've received quite a few "I want to buy your stuff! What's your PayPal address?" emails from zero-feedback members. I've got nothing against zero-feedback members - I was one once myself - but way too many of them send these emails, and are never heard from again.

Even worse are the ones who say, "I will pay you later today/tomorrow," basically asking you to hold the item, and then never come through. I've missed a couple of opportunities to sell because of these people, because I feel that if I've promised something to someone, they should get it.

So, is this some sort of sick joke than I'm too clueless to understand, or is it some sort of scam to build a database of PayPal email addresses for hack attempts, or what?

I suppose I need to start setting time limits, i.e, "My PayPal address is xxxxx, you have four hours to make your payment." I hate to be that way though.

Suggestions? Sympathy? Scorn? :-)