There are two ways to purchase an NFT that's listed for auctions. The first one is by bidding on it and participating in the auction on equal footing with the other bidders. Some auctions may have a "Buy Out" price, that will allow you to purchase the NFT outright instead of waiting for the auction to finish. To participate in an auction, simply press the "Bid" button on the NFT. In the modal window, you can place your bid or choose the "Redeem" option to buy the NFT at once.