I don't think there's an EU-wide site for used items unfortunately. Each country has a couple of local sites.

leboncoin.fr in France, subito.it I believe in Italy, in Spain Wallapop.es although wallapop also exist in some of those euro countries, anibis.ch in Switzerland, I am not sure about the rest and I guess they don't have a public api so you would probably have to rely on web scrapping.

+1 for Wallapop in Spain. There's also MilAnuncios, but Wallapop is what people say when they say they want to sell second-hand stuff

blocket.se, tori.fi, huuto.net or huutokauppa.fi. I think.

marktplaats.nl is the big "eBay-like" for the Netherlands.

kleinanzeigen.de for Germany