I have been shopping lately for a "future" IT Rx-7, and noticed that several used RX-7's say that the are in need of a new apex seal.

Exactally what is an apex seal? (I've had it explained, just wanted clarification)

How hard is it to replace? Are there books/articles on how to replace them?

Would it be worth paying under a thousand if that is (truly) all that is wrong with it?