Good for her and good for you..... People will usually listen unless, like you say they are a jerk, or just really want to get rid of stuff and will sometimes sell it cheap or give it away ...
I went to a yard sale in Mendocino while there, after lunch at Patterson's pub. They said no set prices, donations only. Whatever you want to give. It was to raise money for their kids to go to school.
If they just want to move it quick is fine but usually especially at estate sales the sellers are not aware of the value.
If they are and are still willing to do it then of course I will buy it but if I don't tell them I feel like I am taking advantage of the situation and their loss