I agree about water damage. It is too expensive to fix. Avoid at all costs.
Trailers don't have much that wears out. You can see the tires. Ask about wheel bearings. Look for signs of an accident. Look at the frame and check for rust.
I like to make sure everthing works. The refrigerator is really the only expensive thing to replace.
I would avoid a trailer that may have been lived in. You can tell by wear on the floor and the upholstery. Ask about it.
If you find something that doesn't work uses it as a negotiation point.