It is the area where the umbilicus is but it appears to me to be too long and not properly closed. I believe this is often a result of incorrect incubation temps. I would be cautious here if this is what I suspect there is already issues with this breeder and I would be concerned about any breeder selling a snake with what appears to be a unclosed umbilicus.

This snake IMO should not be for sale, I would question the ethics of any breeder whom had this snake for sale with such an issue.