But where does the liability stop? Did the guy who brought it in to bassdirect know? He sold it as an 83. Did whoever he bought it off sell it as an 83? Truckstop traded it abroad as an 83, there's a chain of people involved in this. Mark didn't magic this bass out of thin air, it was sold to him as an 83 too by a trusted customer. Maybe, as a retailer, he should've done more to check the total authenticity but he has been misled, intentionally or otherwise, too.
Personally, if I were buying something where there is a premium paid on the basis of age, I would do a little homework first and a thorough inspection before paying money purely for the age of an item. This is a crap situation for all concerned but I don't think this is entirely the shop's fault.