It's simple demand and offer. Thanos is still being shipped out so people who bought it to flip are progressively unloading it, same goes for Phoenix, I know for sure there are still batches to be sent out. Offer grows, prices drop.
By the way, depending on where you're located, it seems that pretty much everything is getting harder and harder to sell, with just a few notable exceptions. I just passed on a Wolverine PF that I could grab at $450 shipped, it went on Ebay at $500 starting price and no takers. A friend of mine just sold his Red Sonja PF for 240$ shipped and Batman Keaton for 330$ shipped - consider that being imported, here stuff is a tad more expensive, those two retailed at around 400-420$ in preorder.
