The big enterprise megacorps that build 3D rendering farms tend to only buy new hardware, even if the price-to-performance ratio sucks. The extra capital cost is tiny compared to the labor cost of six figure technicians & developers. At worst, they will buy last-gen Quadros or RTX 4090's.

The ideal market for these used RTX 3090's would be somebody like Linus Tech Tips or a scrappy AI research startup who needs model training for cheap. These types of companies won't amount to enough demand to buy millions of RTX 3090's.

Therefore I'm sticking to my $500 prediction.