A couple of thoughts….
1) I suspect the market for these ‘desirable’ melodicas is relatively small. So that after a few sell for higher prices, the pool pf potential premium buyers is tapped out.
2) I think there are seasonal fluctuations on eBay in many categories, and it seems to me that summertime (when more musicians are out performing, or otherwise away from their computers/internet for outdoors activities), is less active on eBay, in that regard.
3) As sellers get high prices for certain models, more sellers put them on offer there, thereby pushing the supply/demand effect on pricing towards lower prices.