Another one. :(
There is a serious moral disconnect folks have when selling anything with tires. They wouldn't ordinarily steal or rob from their neighbor but when it comes to vehicles, their moral compass is ignored. I've seen this in progress from some fairly decent folks and have been on the receiving end a few times.
Once, I bought a really nice Mercury from a local guy and didn't notice the transmission wouldn't shift into higher gears because the test drive was on crowded streets. Even after asking many times for him to divulge any or all things wrong and that it wouldn't influence my buying the car, he denied any faults. When confronted on the phone. later, his reply was. "Didn't my wife tell you....well it's sold now". I found that the vacuum booster tubing had fallen loose, and once reconnected, the car drove fine. The seller lost more than a $1000 for his dishonesty.
The OP sounds like a handy fellow and will be able to overcome the deficiencies. Maybe a letter written in legal format asking for compensation for damages would be enough to frighten the seller into giving up some refund. Beyond that, court and lawyers is a losing proposition and the OP's mental health would be better served to just fix and move on in life.
The nice thing about fixing the leak problems is that you will know that the trailer is better than new.