Non-MLS deal, who pays broker commission?

[Tyson Hosey]

So, I'm purchasing a property from another investor off market, and the agent is representing both sides of the deal. He had a seller who needed a buyer for his property and I came in as a buyer needing a property. Before we went under contract he sent me a Compensation Agreement to sign saying that I would pay commission on the transaction.

I'm a newbie, to be fair, but I've never heard of a seller paying commission and I've talked to an experienced Real Estate Agent who said in 10 years she's never had a buyer pay commission.

I don't want to offend my agent (who currently manages one of my rental properties), but I feel like this isn't standard. Should I walk away? Should I pay it to maintain the good working relationship? Am I getting the wool pulled over my eyes or just not understanding how the process works?