The commission is paid by the seller from the proceeds of the sale. There is no additional fee from the buyer to the broker.
There will be closing costs and maintenance fee costs- which are negotiable if included in your offer. On our resale purchases, I included my terms to buy (price for the contract, who pays closing costs and amount , if any, of reimbursement for maint fess). If you don't include everything in your offer, it could be assumed that you intend to pay those fully.
In one resale, the price and available points was good enough that I gladly paid the full closing costs and full annual fees. In another, I refused to reimburse annual fees and insisted that the seller pay for closing (most of the current years points had been used), on another I split closing and reimbursed annual fees.
Just do the math before signing any documents to make sure everyone understands what the actual cost of the transaction will entail.
Good Luck!