Used TCs are like used cars. There are those of us that take care of our stuff. If you get one we owned, you'll be in good shape with something used but not trashed. I've moved to only buying used cars because I'm confident there are people out there trading in great vehicles. After a few years of this approach, my theory has proved out. Sure it is more work to find a great used car for a great price, but they are out there.
On the other hand, the technology has changed. I wouldn't personally buy a TC with slides that was built before 2005 for most manufacturers with Chalet being an exception since they didn't exist in 2005. 2005 might not be exact, but I know several manufacturers had a lot of changes before then. For Chalet or other newer manufacturers, I wouldn't buy one from the first year or two. Slides are pretty great if done right, but based on what I've read here, the manufacturers didn't get it right their first time.
However, every TC is "handmade". The quality varies greatly even in new ones. A great delivery inspection is essential. From time to time, there are posts here with people complaining about their new ones that have issues. If you search, you can find that evidence as well.