No matter how many they put out, there's always people buying or reselling higher. Nvidia is overpriced as you said but still available.
I think the only ones buying rx 580 are uninformed miners or gamers now, smart people are waiting for Vega, they think Vega will save them hehe
Actually I do not understand all this hope in the new VEGA architecture. The first benchmark showed 300W power usage and doubt they will do 60Mhash con ETH, which is less that what you can get using current AMD cards