I think it should become customary for people viewing a home to leave some sort of tip to the homeowner. Its not fun to clear out at a moment’s notice for an hour or more. Often the prospective buyer (or buyer’s agent) doesn’t bother to leave any feedback so we’re often left with no clue anyone’s even been here.
You wouldn’t come invited to someone’s party without bringing a small gift or snack, so why should seeing someone’s home be any different? Leaving a tip or gift would show appreciation for being invited into someone’s home, even if you don’t choose to buy.