Hey Kyle, most trailers under a certain weight do not have to be titled in Texas. As long as you have a bill of sale, you should be good. You can choose to get a title when you register it.
To be on the safe side, you can get the tag number and call the tax office to see who it is registered to...you know, just to make sure you are not buying a stolen trailer. They can also let you know what else you need, but I think it is just a bill of sale.