Option 1: The sellers probably don't want to sell them anyways, so they sell it for a ridiculous price just to see if anyone would buy it.
Option 2: They want to make it seem like thanatos cards are worth that much, but no one is probably buying them at that price.
Option 3: People are buying Thanatos cards at that price and you just have to deal with it.
The GMs will never interfere with the economy because if you have a rare item, no one should tell you what you can sell it for. Someone could make a Market Price guide, but nobody should be forced to sell something at a fixed price.