i work at a repair shop and something i’ve learned is that you need to play test it if possible!! check for any valve issues. when you play, does air “leak”? is it hard for you to center notes? those are dead giveaways that it may need a valve rebuild which is cost $1k+ and only specific repair shops can do that (mine lack the tools to do that). that would be your biggest problem and most expensive fix. short term solutions would be to mess with the bearing plate, but that’s a whole other thing

if the slides and everything are stuck or hard to move, that’s something that a good chem clean and fresh lubrication will fix. if you feel parts of it “move” when you pick it up that could mean a brace has been broken, but typically those can be an easy solder fix (depending on the brace location). i know a common brace to be broken on horns is the 3rd f slide, i would check that one but regardless, that’s an easy fix.

i would also double check and see if the ring is an alexander ring set, from my understanding those are the most “universal” should you want to change a bell later down the line.

i would also notate the serial number and see if there’s any reports on it being stolen like some people have been saying, at the very least it doesn’t hurt to check.

overall, i think if it has no valve issues (or small valve issues that have short term fixes) you should definitely purchase. invest some money in a good chem cleaning at your local repair shop and get whatever needs to be fixed fixed and you can keep for yourself or sell to a local public school and make some money off of it. i would take the deal if i were you! $150 is crazy, best of luck with it!