Wanted: AMD GPU RX 580 or better (willing to trade for Nvidia)

I have this Mac mini with thunderbolt3. I also have a thunderbolt3 eGPU case. To make them work together, it requires a GPU, specifically an AMD GPU.

Therefore, I am looking for an RX 580 or better. That includes RX Vega 56, RX Vega 64. It may even include RX 5xxx, if you can convince me that they work with BOINC now (history says it's not good).

I am willing to buy or trade.

No AMD GPUs here.
All Nvidia.