That's a good idea and should be pretty simple. You would need to run a query to find out how many have been sold and then use an "if" statement. Unfortunately, I'm really swamped right now and wouldn't be able to code it for a couple of weeks. I'm actually a little new to PHP, otherwise I bet I could bang it out in a few minutes. If anyone else sees this request, feel free to give us some code. It would have to be hard-coded in the product action sections, otherwise a new field would have to be added and some admin modifications...
If no one comes up with something I'll get to it ASAP. Good luck