Most stores hear several times a month, if not weekly, “Wow, you have such and such. I have been looking for this for a long time.” Looks at the price. “Oh, well I was looking for this for a reasonable price.”
Most customers don’t realize that stores do set items at a reasonable price (after all, we are in business to sell stuff) but that price has to take into consideration both scarcity and demand.
There is a reason why the Dark Tower boardgame sells for $400 on Amazon and Advanced HeroQuest lists for around $200, while a Theros Thassa’s Bounty sells for a dime or less. Dark Tower is scarce, Thassa’s Bounty much less so. There are millions of Thassa’s Bounty cards in print, while no one is making any more Dark Tower or Advanced HeroQuest games (unless Games Workshop or whomever holds the copyright to Advanced HeroQuest decides to do another print run). People who own copies of either AHQ or Dark Tower generally have a pretty good idea of the value of the game and won’t turn loose of it unless 1) they get what they consider a reasonable price for the game, 2) really need the money or 3) find something they want more. Generally stores get in these games under conditions 2 or 3, as purchasing a product #1 doesn’t leave much profit for the store.
The other major factor coming into play here is demand. There is demand out there for Thassa’s Bounty cards, plenty of demand, but nowise enough to absorb all of the cards that WOTC has put into print. Conversely, there is demand out there for Advanced HeroQuest and Dark Tower, much more demand than the few copies available can satisfy, driving the price of those few available copies way up beyond what most people consider “reasonable”. However, if you really want a copy of the game, either because it will fill out an empty spot in your collection or you have fond memories of it from your youth (which is what drove sales of the reprints of 1st and 2nd edition AD&D books WOTC release over the past year), $400 is a “reasonable price” to pay for Dark Tower, especially a working copy of the game (our last copy only worked sporadically and sold for $250).