Does a seller owe a commission without a contract?

[Loren Neufeld]

I recently contacted a local realtor in my area, as a third party, to get a valuation assessment for my father's property. The next day she came to look at the parcel and at that point said she has a client that would be interested in a viewing. The following day the potential buyers arrived at the property, liked it, and the realtor came back with an offer. My question is what is a reasonable commission for the seller to pay with no contract in place?