This may not be similar, but....
A few years ago I bought a crestie hatchling with a bump by the vent from Petco. I knew I'd need to take it to the vet, but it was so loaded with white markings, and the cost of the hatchling plus the cost of the vet would still be well under what I'd pay for a similar hatchling from a breeder. They didn't seem concerned about it at all, and it clearly needed to go to the vet. I decided to chance it. The lump near the tail continued to grow, and after four trips to the reptile vet and weeks of giving it medicine, it died. The medicine had some effect and slowed the growth. We tried two different reptile-safe antibiotics. Hopefully the snake doesn't also have some sort of encapsulated infection in that lump. I'd be wary about buying the animal. It's a different species, but I'd still be concerned. Just my two cents' worth.