There is no obligation for a seller to list on ebay, or any other selling site, at a price based on Beckett, or based on the last comparable sale. The seller has the exclusive right to determine the asking price.
There is no obligation for a buyer, who is interested in a card, to pay the sellers asking price. The seller may take offers, the buyer may make offers.
The definition of "Market Value," at any given moment, is the price a willing seller, not under duress, will accept, and a willing buyer will offer. There is an assumption of equal information about previous sales, values, and relevant information.
When there is an item that sells for different prices, a range of prices, over time, that is an indication of demand, and change in demand.
I work at tables at card shows, watching the showcases for a dealer, who has put asking prices on cards. On a regular basis, young men come to the table and look at a card, take a picture of the card, do some research, and offer 35% of the asking price. When I turn them down without making a counter, they launch into the argument that they are offering 10% under the last comparable sale price. They think they are making a fair offer, and are insulted that they are being rejected. The hoard of flippers only made that offer, because they no that the last comp was an out lier. They have no interest in owning the card for their collection, they are not collectors. Their interest is buying a card that they can flip for a profit.
I've been Occasionally buying cards on ebay for 25 years now - FOR MY COLLECTION. Nothing has changed in 25 years. IMO, The lesson learned is only buy cards at auction, where the initial bid price is $0.99, no matter if it is a $5 card or $500 card. I will gladly be part of a real auction, in which the entire world of collectors, if they are paying attention, will establish the fair market value of a card [or set, or box, assuming no shill bidding.] At the end of the auction, with 5 or 10 or 50 interested buyers, 2 bidders are left standing, with the winner being the buyer who is willing to pay more.
Or buy a card at a show, or ebay with a BIN price, if the asking price matches the value that I place on the card.
Discipline is important to me. If I see a card I want and I think it's worth $100, I'll buy it if the ask is $100. If the asking price is $200, I won't buy it, and I won't make an offer. I operate on the assumption that offering 50% of the asking price is an insult, and I'm not interested in insulting anyone in this hobby. I don't get angry about it. I might put it on my watch list, to see if the seller drops the price after it goes unsold for some time.
There are collectors out there, with no discipline, that must have a card, even if the asking price is outrageous. They pay the price and then complain that they were "forced" to over pay. They were "ripped off." I have no sympathy for the guy that pays too much and regrets it.
"Ability may get you to the top, but it takes character to keep you there." - John Wooden
