Same here, price have doubled if not tripled since I join the movie poster world, even posters from my cheap niche.
Sure, post-Covid but as Tob says, more and more players online, buying and reselling... they actually do thing with almost everything, from sneakers to toy cars.
I call them "MBA Daddy's boys", they buy mostly 80s posters (because Dad loves them) and also 90s and even more recent ones. Then they put it in online shop, ebay, Etsy, Instagram, etc.
And yes, even shops do the same which I fail to understand as their retail prices are way above market prices, but nothing they can do, they have to outbid and play the game too.
Not an easy for shops, it is difficult for them to keep their retail prices fair due to high bids. At the end, things will get back to normal.