A lot of times, the biggest effect on resale value is simply age. Seeing a 2009 M3 vs a 2001 M3 will essentially clench it for a lot of people. Plus the badge can represent a lot of value too, which might be why you see so many cheap Audis. Beyond those, if the car is in good condition, you can drive up the price from the Blue-Book value (Those might just be an American thing...) and remember that whoever wants to buy it will bargain it down, so it is reasonable to up it a little over what you expect it to sell for.
As for recomendations for you as a buyer, it really depends on what car you're looking at. If you've ever seen the Top Gear where they all bought the same make of car and tested it, you know what I'm talking about. I can't say a lot about Audi model quality or anything, which is why Google was invented

Keep in communication with the person you're buying from, and if it's online, ask them to get a dealer check and send verification to you. If you do that, you may be asked to pay for it, though. What it really comes down to is that every car is different, and I personally don't know of old Audis having any aging issues.
Finally, really shop around, because you can get some rediculous deals with old cars that were worth 100,000 dollars in their day, and with aging are worth around 6,000. This was on Top Gear and American Top Gear, and there are old Bentleys and Super-Mercs for very low prices (I don't know what you're price range is, but a Bentley >10,000 is a steal anywhere)
Hopefully this was somehow helpful
