I know some eventers that double as hunters, honestly they usually have one thing going from them right off the bat - a good canter.
Lead changes, and automatic ones…ideally before a corner, are important. He will have to be spot on with these to get $$ as a hunter prospect. If you are marketing him in a lower price bracket, “sticky” changes may be forgiven as long as there is hope for them!
Also don’t think that buyers want a low head for the hunters, self balance is just as important in the hunter ring as eventing. Often they travel on a light contact so self balance is important. They must be able to back off or move up for fences when needed (without the rider piloting them!) to allow for the best jump possible. A horse that will run to the base of a jump may be ok in the eventing world, but in hunterland, they need to be comfortable with finding the sweet spot either by rider choice, or even better, by horse choice.
And lastly, jumping style. Start doing lots of oxers to improve front end. When showing him to potential buyers, set a course of all oxers as these are most frequently what is seen in the ring (or "verticles that are built up so much in front that thy are really oxers!). It will help square the front end and slow the jump, to make him appear more hunter type.