Marketing cost s scale with the price because the higher the price the higher the clients expectations of what marketing should be. Anyone with a house above $1.5M wants to see their home in Dreams Homes magazine which costs a grand or more. They expect professional photographers at 500 to 1000. They expect color display ads in the UT.
As for liability, any agent who has been in the business for any length of time will end up in arbitration/mediation/litigation etc. Even good and honest get sued because of dishonest clients. The comission is built inot the market because the system is what has created a fairly efficient market. Take out the variable incentives and lower the compensation and prices will drop. Most of you would argue that is a good thing….that is except when you become the seller that wants top dollar.
Personally I would love to work for a set guaranteed rate where the client bears the burden for unreasonable expectations (i.e setting too high a price and not selling).