When I read your question, my first thought was that the fee your agent is refering to is a conveyancing fee as opposed to a fee that is paid to your agent.
The purpose of a conveyancing fee for a buyer is to essentially get your transaction to settlement, whether it's coordinating with the mortgage company, setting up a time/place for settlement, following up on U&O or condo/hoa docs, sending out notices to all parties, ordering title insurance, etc. These are all "behind the scenes" activities that need to be taken care of in order to have a successful transaction. $300 is pretty typical for this type of fee. The alternative would be for you to handle these things yourself and have your broker NOT do any conveyancing. For what it's worth, I have not had a situation where a buyer has done this themselves instead of agreeing to this fee or paying an attorney for the same services.