Hi,
When you get the pictures also send them to the person you bought it off.
I wouldn't do it in an accusing way just politely say you have noticed this and are worried and concerned. It's entirely possible they didn't notice, or even that it has happened since it was shipped.
My little guy somehow managed to do that to himself overnight in a hatchling tub containing nothing more than a waterbowl and a light plastic hide.
Basically get as much advice and information about it beforehand and give the seller a chance to make it right if it was an existing condition. Mistakes happen and problems arise - it's how people choose to respond to them that counts in the long run.
It could be the skin folds are caused by him taking a poop and looking deflated and the small lump you can feel is a urate or it could be the "hardbelly" condition described in the kev McCurley book.
The pictures from all angles should let more people weigh in on what it could be.
dr del