Its true that prices vary. But having an extra resource of information can't hurt. If anything it just helps to limit the fluctuation of prices which happens a lot in pro. As for supply and demand you just answered it yourself xD it varies according to buyer and seller. In the end of the day if buyer and seller agree, no matter what the price, its a deal.
Time and date would be recorded in the archive so it actually helps with that not hinders it.
P.S. People who ask for pc in all chat most the time are met with troll answers. And different people give different pc so instead show some solid proof of something that sold for said price rather than everyone's opinionated saying.