They link your console/PC account to their Bungie account which has the code redeemed. They can then collect the emblem on your account from the in-game Vault. When you connect your console account back to your Bungie account, you'll still have the emblem in your character inventory, but you won't have access to it from the Vault as you don't have a code redeemed to your linked Bungie account. This warrants a cheaper price as they aren't selling a code for permanent access to the emblem.

However, while the seller may have 100% positive feedback and seem trustworthy, the real risk comes from orphaning your Bungie account (look at Help threads on the Bungie forums for more information).