There sound never be an issue if you use an agent familiar with Fannie and/or REOs, and you'll be more successful if you do.
You don't pay the commission, and Fannie or the bank already knows they're paying a commission. (and has it worked into the numbers so you won't lose a house because on a commission)
I realized (a little late) my original post may have been confusing. I am planning on purchasing a Homepath property as both the purchaser and the buying agent. I know some banks have issues with paying realtor commissions to the purchaser. I'm wondering if Fannie Mae is one of those institutions.