KBB shows the truck value, in good or better condition, as 14,518-18,615 from a dealer. So the price is good considering you say the truck is in pretty good shape.
The listing says they believe there is a tune on it. So that’s an unknown. There has been a 5th wheel hitch so how much it towed and how it was driven when towing is an unknown.
Kalimax has a good idea about checking the balance rates.
2007 2500HD LBZ GMC Sierra Classic Crew Cab SLT 4WD
MBRP DP Back 5" Exhaust
LB7 right side up pipe