After the NFT project is offered for sale, you can buy or sell it in the NFT marketplace. The are some NFT market platforms as follow:
OpenSea: https://opensea.io/
Looksrare: https://looksrare.org/
X2Y2: https://x2y2.io/
There are many fake programs on the market that use very similar names and even fake Verification badges in their profile picture. If you are looking at a popular project and you find it with a low traffic, pay more attention to it!
OpenSea: 2.5%
Looksrare: 2%
X2Y2: 2%
Project Royalty: Varies by project, usually around 7.5%
eg. When you buy a NFT on Opensea, the price of it is 0.3 ETH and the gas fee is 0.02 ETH. You need to pay 0.32 ETH. When you sell a NFT on Opensea, the price of it is 0.4 ETH and the transaction rate is 2.5%, and royalty is 7.5%. You can actually get 0.4(1-2.5%-7.5%)=0.36 ETH* after it successfully sell.