How do I know what I'm buying is legit?

Check the contract address!
Each store is its own smart contract on the blockchain - which means it has a special address that no other store has.
Each item belongs to a store (smart contract) and shares that same address, and each item has a token ID as well that is unique to that item.
The store's address - there will only ever be one store with this address

How to verify what you are buying belongs to that address:

If you are buying an item and want to make sure its legit, you can check the store's address and make sure the item you are buying belongs to that store.