When I did have stuff to sell I was a bit cheeky and asked Bassbros what they’d offer me as trade in and used that as the basis for sale prices. If I hadn’t sold them in a week or two, I would have traded them in.
For context, I was offered £350 by BB for a 1970s Mexican jazz (block inlays one). I sold it for £375 which was, give it take, what I paid for it. I have seen them for sale on eBay and here for over £600 which seems mad to me.