I’d never pay for the Blue Book when HiFi shark can show me a very long history of item sales -- not what they *should* be priced at, but what historically they have sold for.
That is simply incorrect. HiFi Shark searches the web for listings.
Listings show you asking prices, not selling prices. It is impossible for it to know what an item actually sold for.
even if it did have the selling price, which it does not, it would have the same issues I pointed out earlier
Something could be sold out desperation, it could be sold as scrap, it could be pristine with warranty, it could have extras like cables or tubes. The data set is too small, the condition of the item is unknown, the circumstances of the sale are unknown, and most of all the actual selling price is unknown since it may or may not be accurately reported and may or may not include fees and shipping.