OpenSea is a crypto asset marketplace allowing for the sale of tokens from multiple platforms (including but not limited to Ethereum). At OpenSea.io an artist may upload an artwork and list the piece as an NFT or as a fungible token (similar to a limited edition run of an artwork). While there is a fee and gas cost to initializing a seller’s account, once that is completed a collection (like an artist shop) can be created grouping artworks together and listing them for sale. There are several transactions that are free; among these are minting, changing the price of an asset, and listing them for sale. A listing may be at a set price or may be auctioned off with a reserve price. Artists also have the ability to set the number of assets minted, how to package assets together for sale, and even set a royalty amount for secondary sales of their art.
OpenSea is great for collectors too, as they may find NFTs from other sites such as Rarible, Decentraland, Async Art, The Sandbox, and many more. Purchases can be made through common wallets like MetaMask, Coinbase Wallet, TrustWallet, Dapper, and several others. Current daily sales volume is around 5 million USD making it the number one marketplace. Due to all the flexibility of this site, it may be a one stop shop for most artists and collectors.
*note that I do have a collection listing my artworks here on OpenSea https://opensea.io/accounts/JamesConnArt