I have seen this twice from volunteering at a reptile sanctuary (we get in sick animals often) and one of the red tails that I didn't see had an abscess and our albino Burmese had an abscess.

I also had seen that big lump on a sick Green Anaconda with an RI, but the lump was very squishy from the mucus. He was one of the many snakes from the Texas seizure. He also could've had IBD by the way he twisted his head and neck. We did euthanize him because he did not get any better in our care and I think it only ate once with us.

I'm not saying the snake has an RI or has IBD. I don't have too much experience with abscesses but since I actually had a chance to feel one on the Anaconda it was very squishy from all the mucus in its throat (I guess maybe it isn't an abscess). So if you do decide to get the boa, I would definitely get a vet checkup for medication if needed. But have the owner take it to the vet since it's still in his care and he shouldn't sell you a sick boa.

This would be a lot better and easier if there was a vet on the site who actually knew stuff and told us what we have to do if a reptile looks sick or now LOL