I buy and sell a lot, and have done so for about 8+ years now. From $10-$15 carts to $1000+ sealed across many platforms. And believe me, when you talk about people buying things on credit, you are talking about a very miniscule amount of collectors. I get more people saying "Hey can I pay on payday" (maybe ~5-10% of people I've dealt with) than people just financing it on credit. So all of those assumptions regarding the crash are flat out wrong.
What you do get are life priorities that take place over collecting. You get married, want to fund a honeymoon. You have kids, need to convert game room to nursery. You buy a house / sports car and need a down payment etc.
Or, as we age, your kid goes to college and you see a $20K+ a year bill coming your way. Or you have to pay for funerals of loved ones. Etc. But those are longer term impacts that shouldn't even be addressed now.
Since the overwhelming majority of this stuff is NOT bought on credit, you won't see an tremendous collapse of people selling stuff for pennies on the dollar. Besides, not sure if you noticed, but if you list a bunch of like items at 0.99 auctions (which firesellers would have to do), they tend to do well and come damn near (or beat) BIN prices anyway.
I do think prices will taper off at some point as our generation ages and collecting shifts behind other life priorities, but expecting a full on collapse is wishful thinking.