For me - the first and most important thing about a yard sale - help me get to it.
If I can't figure out where it is - I obviously won't be buying anything.
I think really valuable items should not be put in a yard sale - just in case they get broken or "develop legs."
I will buy pop or water or food at a yard sale - but I won't pay much more than I would at a grocery store for it.
$0.50 to $0.75 is as high as I am willing to go for a can of pop/soda or a bottle of water.