If I did know, I wouldn’t need to consult them. Though, I’d imagine they use a similar technique to how I would appraise the value of a gaming PC, not that I’m an expert on consumer hardware pricing, but I’m fairly knowledgeable. But I would use historical data, performance metrics (although this wouldn’t apply to retro games), market conditions, conditions of the parts, and a certain amount of speculation.

The reason I’d look for a consensus is to account for outliers. If one expert is using some metric that causes them to over or under value a game, I’m less likely to get ripped off by having multiple expert opinions forming a consensus than if I relied on just that one expert’s opinion.