Apex Seal

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?

For simplicity sake the apex seal is the rotary equivalent of a piston ring. When they break they generally wipe out half the engine in the process. It requires a complete rebuild to replace one. And like a piston ring you don't replace just one and you really shouldn't use new rings in a worn bore so new rotor housings and new or surfaced side irons are the way to go.

If you're building a car or buying a built car with a blown engine it's best to just plan on starting with a completely fresh engine to the tune of about $2k+ depending on who does the work and what options you choose. There is rarely much to salvage (in dollar value) out of a high mileage street engine.