Too many people buy with the thought of reselling, and when there is a steady supply, the value will always stay in check. When enough collectors love a piece enough to keep it in their collections, supply dwindles and fewer pieces are available to those who missed out and are forced to the secondary market. You have to start offering more and more money to convince people to part with a cherished item. But with so many people buying to sell, supply will keep up with demand and ironically the value will not skyrocket. It's the pieces that collectors buy to keep (and then refuse to sell) that become valuable.
People bought Joker PFs thinking that they'll be worth a fortune on eBay. Now there are a huge amount being produced and hundreds of them will flood the secondary market for years to come. The more flippers/resellers hop on board, the greater the guaranteed eBay supply and the lower the value. The sellers start competing with each to have the more attractive price and it becomes a buyer's market. Ten people will put a $300 piece up for $1000, and nobody bites. Then a seller puts it up for $25 over cost and the market price is set. Supply outweighs demand and the prices are kept low. It's an ironic thing to witness.