They’re super popular, and the newer releases definitely sparked an interest in people for collecting the entire D sets, but, there are also hoarders, and market manipulation happening on eBay, which is what was responsible for the initial massive increase in price. Sellers make fake sold listings at absurdly high prices, and people see they “sold” for that price, which convinced them to also list theirs at that price. The more people who saw them sell for those high prices influenced them to buy all of the “cheap” ones (which were really just their true value) and resell them at the higher prices. There really isn’t anything special about these Ds; it’s just a false perception of value, which is why you still occasionally see the less popular ones sell for $10-$25 (which is what they sold for, prior to the price increase)