"Just because something is old does not diminish it's worth. Have the bow checked out for flaws, shoot it. then decide if it is worth it to you or not."
Depends on what you mean by worth. In terms of dollars, an eight year old Mathews is not worth as much as a five year old Mathews. If the guy is selling it at the price of a five year old Mathews, I would say try and find a real five year old Mathews for the same price and shoot them both. If you like them both, the newer bow is the better deal and likely has three less years use on it. Not saying the newer bow is better, but with all the used bows out there, there is no reason to overpay for the older bow. Or find the same older bow for less money. Lord knows the saved money will be used. Arrows, broadheads, release ...
Respect - don't take it, unless you are willing to give it.
Responsibility - don't give it, unless you are willing to take it.