I've been down this road a few times with the 7th gen and the best course of action is to send it out for refurb/repair. There are services on ebay that can fix issues with the radio and display, CD changer, etc. The issue with buying a used replacement unit is that you don't know how much life is left in them before they go again. The cost for repair is about $80-$120+ depending on the what you want fixed (so comparable to buying a used unit) but the repair is warrantied. Reach out to a seller that provides the repair service, tell them what's wrong with the radio, and send it out for repair once you have a price. You would be without a radio for about week (tops).
For the aftermarket units, I genuinely don't trust their quality and support is limited.