Everything in real estate is negotiable. Generally, when selling real estate, the seller pays the commission, however, I have seen situations where a buyer will retain a real estate agent to represent them and agree to pay the agent a percentage of the purchase price if the seller does not. It works out the same for the seller because the buyer is simply going to reduce their offer to compensate for the fee paid to the real estate agent.
When renting real estate with a real estate agent it's even more ambiguous. Here in New Jersey it is not uncommon for the tenant to be required to pay the entire real estate fee. However, just as with sales, everything is negotiable and tenants can propose that the land lord pay the fee, or that the fee be split between the landlord and the tenant. I always tell the landlords I work with, 'pay the fee and you will be on the top of everybody's list', but in a corporate relocation situation the relocation company tends to reimburse tenants for the fee so it's far more common to ask the tenant to pay the fee and then negotiate.