If you've used your trailer for a few years you probably know everything that's wrong or at least have suspicions. Soft floors are easy to detect. They probably did not happen during your ownership unless you've seen a leaking window.
Generally I like fixing up the trailer to make it ours. If you need a new axle
you'll know when you take it on a rough road. It's usually not too expensive and usually most significant locals have someone capable of replacing it.
The key strength of these little guys is their weather integrity. If that's intact you're probably in good shape. Unless you intend to replace it with a new unit, you are liked to have some issue with what ever you buy.