Cardano currently doesn't have smart contracts which makes it a little harder to automatically transfer assets. You could send ADA directly to an owner of an NFT that you are buying from but there are no guarantees that the owner will release the NFT to you. Upcoming platforms such as https://www.cnft.io/ will allow you to resell your NFTs to others that may wish to purchase them. Unfortunately, we will have to wait for smart contracts to appear later this year to ensure assets are moved safely and smoothly. For the time being, there is a CNFT Discord server where you can participate in channel auctions and allow an escrow service to provide the transfer of ADA for the digital asset that you may be wanting to sell. There have been many NFT pre-sales on Cardano lately. Some have been highly successful, some have been plagued with issues. Many of these issues smart contracts can solve.