Somewhat ironic that popular MtG podcast Limited Resources is sponsored by FTX (and popular card marketplace Channel Fireball).
Europe is usually CardMarket. In the US TCGPlayer seems the most popular with CardKingdom behind them. I've used both TCGPlayer and CardKingdom on the buy side without issues, but you can also list cards for sale (no idea what the process is like).
If you want to get maximum value in exchange for doing more legwork and probably assuming more risk, Ebay or Facebook groups.