Hard to say what’s under the primer. It could be some
fiberglass damage, which may or may not have been properly repaired. Could be some kind of patch over a vent that was removed. Can you see and reach the inside behind the primered area? What account does the seller give?
Some pictures might help.
Fiberglass is repairable, but unless you are willing to do it yourself, it can get expensive fast. Same with repairing the floor and/or frame. And that’s on top of lots of smaller things that will inevitably need attention in an older trailer.
The market is crazy right now, and trailers are selling for thousands more than a few years ago, even projects.
Is there someone that can be a second pair of eyes when you go back to inspect the trailer, preferably someone that’s handy? As hard as these trailers are to find, it’s easy to become emotionally invested in a purchase and end up with more of a project than you realized.