Ebay is a really good resource.  Just search for RX 580 and then select the option to show sold listings.  from there, you can get an idea for how much they are selling for.  Exception to that: you may see some ridiculous low prices that are scam attempts and invalid.  Judgement call on your part to determine what's a valid price but it's usually not too hard to figure out.

Personally, I just did Craigslist and sold locally and I'm in a weird/limited market place... sort of geographically isolated comparatively but it all worked out fine.  Good thing about CL compared to Ebay is you don't have to worry as much about fees, packaging, and returns.  Just don't be a jerk and sell crappy gear or else I can imagine other bad things happening.  I've heard stories. 

But yeah, I really like CL lately.  It just depends on the margin between sales and hassle between various resale options.  I was really pleased to use some used GPU's I picked up off ebay and then sold them for a little bit more after mining with them for several months.  What could be better than that?  lol