On one hand, you might look at the sales commission as a lot of money that you don't really feel that is deserved, but on the other hand, if you are working with a really talented agent that truly cares about you and your next chapter in life, the sales commission is quite worth getting you there in quicker, with less stress and more ease!!
Do what feels right and establish a good working relationship from the start! Sorry to go off on a tangent, but I just feel like these things are more important at times than negotiating commissions!
Personally, I charge a lower commission of the seller is also buying, and I charge a lower commission if the buyer doesn't have their own agent. Every agent/agency has their own policies. Let me know if you'd like to discuss this more.