Old ravens flatbed questions

You can take anything I say with a grain of salt or totally ignore. Your choice. You sound like me, have enough knowledge to be dangerous but cautious nonetheless. I know a lot of things, but I don't know everything, and that's where I start my evaluations of equipment. I learned a lot, by taking a 'company' trailer to a reputable repair facility, that the on-road people didn't know was off limits to us to use...nonetheless. I picked their brains and asked questions, learned what to look for, etc. This is what I suggest to you. Find someone that specializes and can inspect that trailer for you, based on your criteria, and ask their professional opinion. It may cost you a little, but better some now than more later....and let them do a thorough inspection FOR you. If the seller hedges allowing that, no matter how legit you think he is, that's a big flag. Your trailer mechanic can also give you a better idea of what a 'good' price would be.

If money/budget is a problem now, what will it be like when you have to have repairs on the trailer also, that you now own? I'm not a Debbie Downer, just a practical risk taker, but I know my limits and that's when I have to get someone whose opinion I can use for input, no matter what my gut DESIRE is to do. I'm not to proud to be talked out of what I want to do, if there's good reason. I sense from your questions, you don't have the mechanical chops that might be required to ascertain the true cost of this trailer.

If you need a second mechanic opinion, and believe me, even experts can be wrong, (I just repaired something that they got wrong), so get a second one. It sounds like you're in need of advice that someone you pay to protect your interest, could be of value in this instance.
Good Luck, but the fellas here that have given you some advice, actually work on the equipment. @jamespmack and @OLDSKOOLERnWV in particular are helpful knowedgeable sources of information and you would do well to heed their advice.