flat fee listing service and selling on terms

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I have a house that has quit a bit of equity that I'm looking to sell. In the past I have sold a lot of my homes with owner carry. I'm considering doing a flat fee listing.  

I'm fine to cash out of the the property or sell on terms. I'm willing make a cash buyer a great deal, but I'm not willing to sell on terms at below market price. I'm trying to wrap my mind around how to structure it. If I  put the list price at the cash price then it would be hard to raise the price if a buyer wanted terms. 

 If a buyer's agent brings me a cash buyer that would be  lot different than them bringing me a buyer that need  me to finance the sell. For example 3%-4% of the sells price my be more than the buyer's down payment. Has anyone ever offered to pay the buyer agent 20% of the down payment instead of 3% of the selling price this is if it sells on terms?

Also does anyone have a recommendation on a flat fee listing service for the San Antonio, Texas?