All of my kegs were purchased used, and I really haven't had any issues. As long as you're buying them in-person, you can inspect the interior. I've gotten most of mine from the LHBS and/or the place I get my gas. I also got a few from a fellow brewer.
The outsides may be dinged or scratched, but that doesn't impact the beer.
Based on the price differential, I can't see buying new 5 gallon kegs.
The only exception I would make is if you are interested in 3 gallon kegs. I bought a couple off e-Bay a few years back and after having to replace gaskets, poppets, etc. I was right in the ball park of buying new. If I was to go for more 3 gallon kegs, I'd likely go the new route just to avoid any potential problems since you can't usually find these used at the LHBS.
I do keep extra gaskets around, but haven't really had any fail on me. I've replaced a poppet or two and pressure relief valve or two over the years, but no real maintenance other than cleaning.