I'm planning to build a camper. Saw some used trailer frames on craigslist. I'm concerned that I might buy a frame that I won't be able to use.
You need to consider how much it will weigh. When built. Will you need just one axle? Or more. Look for rust, cracks along the frame. Look for anything that might be bent.
about, You'll want a frame that is free of rust through spots. Surface rust can be sanded and painted. Also, you'll also want to find out when the bearings and seals were replaced last. I don't know how large of a camper project your planning. Just be sure to get one big enough based on GVRW= Gross Vehicle Weight Rating. The weight of the camper material plus the trailer weight, and then get a little higher GVWR, rating of the frame. Good Luck!