I know I've beaten this analogy to death, but I believe it's still applicable to this day (and carries with it the same weight in terms of importance):
You wouldn't buy a used car if the owner wouldn't supply the keys... and you should not purchase a computer if the owner is not willing to supply the original install/restore CDs/DVDs that came with the computer.
Buying a computer without the original install/restore discs is like purchasing a used car where the owner supplies you with only a picture of the keys.
The original install/restore discs that came with the computer are your ticket to general troubleshooting, reinstallation and maintenance. While the computer may function fine without them, the day will most definitely come when you need those discs, and then you're up a creek without them.
On the other hand, the site I linked to above (lowendmac.com) and the deals you'll find through 3rd parties will most likely NOT supply you with the original discs that came with the computer a lot of times. It is up to the buyer to acquire a set of discs (and this is another reason I recommend purchasing an Intel Mac over a PowerPC Mac), which can be purchased through Apple's online store. If you purchase a G4, then you will need to locate a retail copy of Leopard -- which is difficult to do for a reasonable price (no one sells it anymore unless you can find it used, and even then, it's a crap-shoot on what you're going to get). If you purchase an Intel-based Mac, then you can simply pick up the newest retail version of Snow Leopard, sold throughout the world and easily had.