I've been looking on eBay but can't see any change. I'd like to buy a lot of GPUs but don't know where to look
This has been hands-down the most ignorant view of the ETH-merge proponents. The floor of the currency is tied to the cost to mine. There's minimal cost to mine now. The price will fall. It's not a supply vs demand problem.
This is not true because of difficulty adjustment. If the price drops enough so miners are losing money, some of them quit, the difficulty adjusts downward, and the economics improve for the remaining miners.
Broadly: what do you do when you can't go to Cancun like you planned? You bet on Doge and GME, apparently. (Or you declare yourself a "VC" and start handing out checks to 20-something quarantined hackers.) Then you just end up with more money you can't spend.
They also have more or less real-time pricing and price history for individual products. But it doesn't provide visibility into the used GPU market.