I recently sold my RX 480 and honestly it was pretty difficult to find any equivalent GPU, new or used, at a decent price in the $180-400USD range. When the 480/580 supply started drying up the miners just started gutting both the primary and secondary market of almost everything in that range. GTX 980 Ti and 1070 models were the first to really dry up because of folks selling their GPUs and buying into those, then miners started grabbing them, then 1060 6GB models, then 980s, then 970s and now you can't even find 1060 3GB cards.
This is just a terrible time to get a GPU below the 1080. Be patient. GPU manufacturers will match market demand to the best of their abilities but they have to be cautious to avoid a market crash leaving them with tons of unsold cards, hopefully we'll see stock improving in the next month or two and once the price on the 480 and 580 come down the market will settle back into more intended pricing.