In Europe they have always been around EUR270 before and after the mining craze. So I am not going to earn anything if I sell it. :P
The mining craze is almost limited to the US, where everyone is used to low prices for electronics haha.
Well i have seen 580s being sold for 400 euros in ebay but if you are happy, i am not to judge :)
I have seen they are selling for 350-400€ now. But Ethereum mining caused a spike also on GTX 1070s, as they are selling for 500€ (1060s are 350€, not worth swapping the 580 for a 1060). By selling the RX 580 I wouldn't get enough money for a new card.