turf, everything is negotiable. And, every tradition or custom (including 10%) has its reasons.
You want the boat or not? You've got three choices:
1-Give em the 10%
2-Give 'em a check for $2500 or $5000 or whatever you think is fair, and tell the broker your offer is good for 48 hours. In today's market he'd have to be nuts not to at least ASK the seller.
These are the ways things are done, and "why" really isn't going to give you any further options, is it?