At the moment, the AMD Radeon RX 570 and Radeon RX 580 are very hard to get as miners are buying up all inventory because at current prices the pay-back time of these cards is just six weeks or so.
There's also increased demand for NVIDIA cards like the GTX 1060 and GTX 1070 but AMD cards are more efficient for mining. The Radeon models have a higher hash rate and are thus more profitable for Ethereum mining.
Because new cards are hard to get, prices of used cards have shot up dramatically. Hardware.Info reports that in the Netherlands, second-hand Radeon RX 480 cards are sold for 320EUR to 350EUR, which is more than the original launch price of 260EUR! Similarly, used Radeon RX 470 cards are rapidly sold for 300EUR.