Each collection on Galler has its own Order Book on the right hand side of its collection page (assuming the owner who imported the collection has activated the Order Book feature).
If you're interested in buying through the order book:
1) Find an acceptable listing in the "Listing" section of the order book
2) Hover your mouse above the listing and click buy
3) Approve the transactions and sign your wallet
The listing amount + associated gas fees will be deducted from your wallet, and you'll be the new owner of that NFT!
For more information on the Order Book, please check these articles: