I like to assign my properties that I am wholesaling. There is the consensus that a buyer may not like the deal when they see you making some money on it, but the fact of the matter is, if you are selling them a house at the numbers they are looking for, they should not have a problem with it. If they do, try to find another seller.
Bottom line is this. If the numbers fit for your buyer, they should have no qualms about what your assignment fee is. You just have to be tough and stick through it. If you are offering houses at a great price to the end buyer, it really shouldnt matter what you are making on your assignment fee.
I have a buddy of mine who found a house for a GREAT deal. Why he didn't keep it for himself I dunno, maybe he just doesn't want to rehab or cant get financing. Anyways, the house was a fire damaged house, needed around 50k in repairs, ARV of around 245,000, got the contract for $50,000, he assigned it for $35,000.00. He sold it too, he got the $35,000 and in the email he sent out to all the investors, he was upfront and stated he had the house under contract for $50,000 and is assigning it to the end buyer for $35,000, but because he had given the end buyer a great price, he got a great assignment fee. This is a true story, back in Sept of 07.