I have a feeling that you will find basically its "buyer beware"... I hate to say it, but you should have pulled the mats up before you bought it, if there was too much rust, then it was there when you bought it. I looked at many many trailers before we found ours and many times, even though the sellers said "the floors are great, very little rust, good shape" when we looked at the trailers , they were no where close to being what the seller said and we walked.
Even if you saved emails, he can say" in my opinion the floors are fine, etc and she did buy the trailer after seeing it. Unless you have something in your state that is called "buyers remorse" and can return something within a few days for a return of your money, then I am afraid you are stuck with the trailer.