I have noticed a drop in the quantity of material for the regular daily searches I do, but
ebay is so large that I don't know if that is a trend or just some anomaly. That said,
ebay has always been weird; I just picked up an Algerian set I'd have happily paid $38 for and was the only bidder on and got it for $15, and other stamps seem to go well above 1/2 Scott or higher with 10+ bids.
My answer is: Who the heck knows!
Sure there is some data out there, and would not be surprised if inflation is starting to maybe bite in to discretionary spending and thus less sales/interest?