I think it started with Tuff Stuff, morphed to Beckett, and is slowing transitioning to PSA Graded guide. I haven't referred to a price book in several years. I use to buy them to read the "Hot List" top moving cards every month. By the time you read the list, it had changed.
When you ended up owning one on the "Hot List" you really thought you had something! 2 months later you're depressed because the card fell through the floor!
As a seller, I think one should sell, or buy, the card that they're comfortable with - maybe too much - maybe a steal - only the buyer/seller knows for themselves. Remember when a buyer, or seller - you choose, would say "Beckett says......". I don't hear that very often anymore now that cards are bought and sold quicker than stocks. Heck, now they can go into a vault and you can resale without even seeing the card.
I can still some use for a guide, especially if one is selling commons, etc. If I remember correctly, the higher dollar cards weren't priced, they were just noted as SP, SSP, not enough sales to determine value. If someone is spending 1K+ on a single card, you can bet they have researched everywhere to understand the current market for that card.
I don't want to condemn anyone for using a reference they are comfortable with. Just remember in today's instant search world, they're many reputable alternatives to just a singular "Blue Book".