NFTs are usually based on the Ethereum that’s the second popular digital currency after Bitcoin. The problem with Ethereum is that currency generation has been using up electricity thus indirectly contributing to greenhouse gas emissions.
In addition to the climate issue, buying transactions are resource intensive and are hence very costly. In some cases buying costs can be a significant proportion of the cost of the NFT itself.
My NFTs at OpenSea use a newer currency called Polygon, based on Ethereum, that uses significantly fewer Ethereum transactions. As an example, no Ethereum has been used yet to offer the initial NTFs to you. This provides buyer cost savings and helps save the planet.