If the agent is charging 6.3% then there may be a reason. You said he brought you another deal. Did he not make much off the deal and did a bunch of work??
If that was the case I wouldn't see a problem paying the 6.3%.
As far as marketing we have a saying in the business.
"No amount of marketing in the world will sell an overpriced listing"
The amount you are asking is relative to many factors.If everything is ready to go with the property then it is just price. If staging wasn't done,or only had a half rehab etc. then regular market comparables won't work until you fix those issues.
If the investor wants a crazy amount of marketing then the broker/agent can have a clause where the seller spends the money and the receipts are taken off of the commission at closing.
This prevents the seller from overpricing,pulling the listing early,blaming the broker for things the seller should have done before marketing it staging,fixing repairs,updating the property to today's standards,etc.
I am an investor as well but what I get tired of hearing is the blame game. If truly the broker/agent is responsible for something they should take blame for it. I find a certain amount of the time the investor likes to blame the broker/agent for mistakes the investor made.
If people focused on what they are getting for the commission charged instead of finding the cheapest rate then you would find VALUE.