I work as the all-around IT guy in a company of 25 computer users. When friends ask me about buying somebody else's computer, I ask them a few basic questions:
1. WHEN WAS THE COMPUTER BOUGHT/ASSEMBLES BY THE SELLER/USER? Bear in mind that what you are buying is only the remaining life of the computer. Assuming that your friend has purchased only best-of-breed parts (either with international quality certifications, several years of warranty, or product awards by reputable computer magazines), I'd say the life of a brand new computer should be 4 to 5 years. Since it's better to be conservative, assume the computer to last only four years. If the computer has been bought 2 years ago, then logically you'd have only 2 years left to use it. Sorry to be stating the obvious, but I often see people buying computers at a "good price" (say a fourth of the original price) when in fact, what they're buying is already towards the end of its estimated useful life.
2. ARE THE PARTS BEST-OF-BREED OR LOW END? If the parts are low-end, assume that the life of the computer to be only 3 years. From there, compute the remaining useful life.
3. DO YOU KNOW THE USER? If you know that the seller/user takes good care of his computer (like if he/she uses a surge protector, uses a UPS, doesn't move the computer while it's turned on, etc.) then you'll be more certain to get the maximum value out of what you're buying.
Buying from someone you don't know, increases the risk of buying a mishandled computer. If I'll be buying from an unknown source, I'd reduce the value to about 2/3 of my initial computation (brand-new value minus used life).
4. IN THE EVENT OF MALFUNCTION, WILL THERE STILL BE REPLACEMENT PARTS? Specially when it comes to memory, you may not be able to find an SDRAM replacement a year or two from now. So, why buy a cheap computer that you can't repair?
If you can have a copy of the invoice of the computer being sold, that would be the best gauge of value. Otherwise, have the seller write down the specs of the major parts. Afterwards, show the list to some IT-savvy friend of yours to see how durable the parts are. You're friend will most likely know when the parts came out in the market thus giving you a good estimate of when the computer was bought or assembled.