Depends on the market. In a soft rental market seller writes the check, in landlord's market you write the check. I've written brokerage fee checks myself when I used to rent (Brighton, Somerville). Fees are ALWAYS negotiable. What I'd recommend if you have not already written the check is to say you want the place but the seller can pay the fee, if the agent objects and you feel you are in jeopardy of losing the place and you want it, tell him you will pay half - again it's all a negotiation, but to answer your questions, yes, it is standard practice.