The simple answer is Supply and Demand. Older Harry Potter Pops may not be produced any longer, but the supply available to meet demand changes as someone like yourself decides to stop collecting HP pops and puts them on the market. You have added to the supply side, and taken away from the demand side. Prices tend to drop when enough collectors do that.

Perceived value also effects demand. You mentioned that you are unsure of paying more than retail if they won't hold their value. You have now effected the demand/value side. This would be a completely different post/discussion if your "collection" had gone up more than 50%.

Why did the values start to drop in the first place? People burnt out/move on to something else? Over produced Pops/lines? The Fad is Fading?