6% commission. Then you can write your offer however you want, but that doesn't change the seller's obligations to his broker. If the listing agreement is for 6% and you write your offer representing yourself the seller is still obligated to pay the 6%, it would just all go to his broker. What you are really hoping for is for his broker to accept less, which they are unlikely to do.

What you should do is tell the listing broker that if they credit back half of the buy side fee to you at closing then you will let them represent you. That way you stand a better chance of getting 1.5%