It's quite common for an REO to have requirements when it comes to who generates the preapp letter. They may require the letter be from a direct lender/bank or even from a particular agent of particular bank. Usually they require this letter to be submitted with the offer, though ultimately you can use whatever lender you wish to work with for the loan.

Beware - If you are shopping the REO market and qualify through five different banks (are there that many left?) to make offers on different properties, each qual process will hit your credit score. If you start with a 720 score, it may reduce your score in the end to below premium rates! So choose your REOs with caution and don't shop lenders/brokers too freely.