Is there a standard royalty rate for licensing a domain name? Who determines the percentage of a royalty rate for licensing a domain name? Or is it what ever is agreed upon between the two parties?
There is no standard rate. If you own a domain name containing a trademark and the trademark owner approaches you to transfer the domain name, don't try to "hold up" the trademark owner - that would be "cyber-squatting" and the domain name can without payment be taken from you involuntarily.
The above is general legal and business analysis. It is not "legal advise" but analysis, and different lawyers may analyse this matter differently, especially if there are additional facts not reflected in the question. I am not your attorney until retained by a written retainer agreement signed by both of us. I am only licensed in California. See also avvo.com terms and conditions item 9, incorporated as if it was reprinted here.
The parties can pretty much negotiate as they will, assuming that it is a straightforward transaction. The question is what the market will bear or what others are doing. That is, a licensee will only pay so much or it makes no business sense, and the licensee will not want to be abused financially. If I were to hazard a number, I would suggest 5% of sales, but defer to others who may be able to be more precise.