Sorry OP, but I have to agree with the majority here.
You went and looked at the trailer and had every chance to say, 'No thanks, I don't think it'll suit me.'
Instead, you bought it. Now that you've taken it home and discovered it has issues, you're out of luck.
You can't expect much from a $700.00 trailer. I paid $1,400.00 for mine when it was 22 y/o. I bought it from a dealer as a used vehicle, and even from them didn't get much of a warranty. Why would you expect a warranty or buy-back clause from a private owner?
The trailer is yours. Either put money into it to make it safe to haul horses, or scrap it. Either way, it's your headache now.