I'm in the US and I use:
https://reverb.com/ (this site is great)
Ebay for cheaper/noncritical items where a bad purchase won't cost me (you can pretty much skip buying synths on there)
Gearslutz (a bit more reliable than ebay but I prefer to use escrow for large items)
Craigslist (always for local purchases where I can test the product personally, and never at either person's home or business unless they're a known quantity).
When it comes to computers I actually do some forum trading/purchasing and we use sites like beerology & heatware for 'reputation', which is far more trustworthy than modern Ebay ratings.