Yes, the seller is able to do that, the new confirmation key won't expire upon using an older one. In fact, the seller can do that untill the registration of the account changes to the buyer his information, so 30 days.
The buyer can check in the payment history of the account when new recovery keys have been ordered though.
What I am curious about is if the old confirmation key is still valid if a newer one is used. So once I receive the confirmation key that I ordered on the 23th of July I will use that one. After that, I will check if the one that I ordered on the 16th of July still works.