Of course GTX 1070 is better if you can get it at this price.
Personally I would not make an investment of this amount in an Ebay product without guarantees or possibility of RMA. (aside from the fact that since the product is in the USA, shipping to UK + taxes can cost $ 100 or more).
However I see that, in the UK, the AMD RX 580 8G has dropped a lot in price and is at a much lower value than a few weeks ago, with what falls within the indicated budget.
I think the RX 580 8G is somewhat better than the GTX 1060 6G (but since it was at crazy prices I did not even consider it). Consume more and need more airy equipment. It is not suitable for a mini-itx pc. But it is an option to take into account.
I recommend that you see some review of the GTX 1060 6G and RX 580 8G and you decide according to your needs.
Good luck in your choice and enjoy your product!