I would inquire about her feeding. What does she eat? From that picture it looks like she is being fed live rats and has sustainted multiple bites. Or she is being improperly handled or housed (bad shedding?) causing scale damage. That kind of damage can lead to internal infection and months of sheds to completely disappear.

Get more info on her from the seller. If he is unwilling to be honest with you about her condition, feeding habits, housing etc.... Then he is not someone you want to deal with anyway.

As for an offer.. If she is healthy. Id say around $1200-$1400.