It is a "free market" - only takes one motivated buyer to cough up the coin for that seller. Really has nothing to do with what a next buyer might pay to the next seller for next item. For example, the serial number might match up to be the rifle issued to the buyer's GrandPa - some would, and are able to, pay a lot for that. Most would not be willing to, or are not able to pay, as much?? Can't say I put a lot of weight in a "going rate" idea - how badly does seller want to sell; how badly does buyer want to buy, and is buyer able to pay??
As the saying goes "money talks" - in this case, goes to a notion called "effective demand" - want to buy, and are able to pay.