Probably been asked before, but couldn't quickly find an answer. I live in Finland, so one located here would be the best, but I'd like to know any.
In Finland the obvious place to find used gear is https://muusikoiden.net/tori/ (https://muusikoiden.net/tori/).
Another good source for used gear is https://reverb.com/ (https://reverb.com/). There prices are often "collector prices" and apparently there are plenty of scammers as well but you'll find good deals if you are patient enough to wait.
In Italy
Mercatinomusicale.com
You'd be surprised how many good deals can be found on Vinted as well, although I don't know if it covers all of Europe. I tend to sell and buy a lot on that platform since a couple of years. Never had a negative experience this far when it comes to buying used gear like pedals or eurorack modules, although i wouldn't necessarly take the risk with more complex gear like synths