Trades happening off market is also due to valve not having a way to cash out. If you sell your $20,000 item on the steam marketplace that's a lot of games you can buy, but they won't send you money.
FWIW, people were purchasing Steam Decks with the "Steam Money", then reselling the console for cash.
Its still not "cashing out", but I'm sure some made some decent money. I would assume you could sell game keys to those less-than-reputable sites as well? Dunno
Agreed overall though, these are just extensions of "happening off-market"
