Buying used or new for that matter can be a mixed bag sometimes. The camper we have now was 2.5 years old when we bought it back in 2007. And the dealer we bought it from was 500 miles away. I knew I was never going back to them.
A few things to help when buying used, these even apply to new for that matter.
1. Educate your self on the brand you are buying. Research the make,year and model. RV.net is a good place to start. People talk about their campers, good and bad. While you cannot take all that is said as gospel, If you get many of that same comments, that is a heads up to dig into the one your looking at. If you have some knowledge going in on the brand and what to look out for can help save you.
2. Neglect. On a camper neglect can kill any good camper quick. The worst is water damage. Before buying any used camper with no long period warranty you have to educate yourself on where to look for water damage. I see ScottG posted on this. He must of been through it before. A water damaged camper can turn a good day real bad and unless you know where to go looking, you may even walk right by it and never notice it until 3 camping trips or a year later.
See this video, forget the sale guy pitch but see what he is looking at.
http://www.youtube.com/watch?feature=player_embedded&v=xx0SPexRHgo Research water damage on a camper, there is a lot out there on web about it. And it does not matter if the camper is 2 years old or 10 years old. Many people have no idea they are suppose to be a hawk eye and fix roof sealant immediately.
3. If you are not handy in all things camper related, hire someone to inspect the unit for you that is. It might cost you $100 to $200 but once you own it, it's your problem.
This all boils down to, educating yourself on what you are buying. There are some really good deals to be had and some really bad ones too. The key is knowing how to spot which ones to walk away from.
In my case I knew the brand and I knew the camper. I knew what to look for and may have drove the dealer a bit up the wall with my inspection requests, but he and I both knew once I paid for it, it was mine. I got a good deal and a great camper. I would of bought new however my brand was one of the 1st to go under before the economy tanked. After searching all over for new, I gave up and bought a good used one on the brand I knew was good. Then just had to make sure the camper was not neglected.
Good luck and hope this helps
John