If it were me, I'd just have it fixed by a good welder and use for a while. Going back to the seller would be the last thing on my mind. I've purchased a few things in my life that seemed fine, only to have issues down the road that I didn't discover at the time of purchase. That's just how it goes sometimes.
I had a scare last year...
I was looking for a nice dozer for my ranch. Looked at nearly a dozen before I found "the one". Checked it out as best I could, test drove for 15 minutes, and wrote the man a check. I got it home, started to push some dirt and noticed it had almost no oil pressure. I'm sure seller had to have known the oil pressure showed low, yet he didn't reveal that to me. Long story short, it had a bad gauge. I never really blamed the seller, I blamed myself for not doing enough inspection. Fortunately I ended up with nice machine for a great price and I think that was partially due to the seller thinking the engine was going to grenade....