"Can I use their bathroom?" The answer from me has always been YES. You can use the home sellers toilet. Home sellers should be prepared for buyers, their agents, the home inspector, the appraiser and just about ANYONE else who enters the home to "use the bathroom."
Home sellers should leave adequate toiletries for their 'guests.' The only time I've ever said no was when the home was in foreclosure and the water was turned off. Then you should go elsewhere. Obviously.
It's a necessary issue to deal with when you sell your home.