The most known and biggest platform where to sell your ETH NFTs is
Below you can find all the resources needed to start buying and selling on Opensea:
General guide
Getting started
Selling guide
Buying guide
Remember that in order to buy and sell you should pay gas fees. The suggestion is to always wait for the fees to be low enough not to impact your final price
Also remember that OpenSea has fees, so do the math before selling
