I think it's also a little based on the culture of the place where you live. For example here in Italy "tipping" isn't a part of the colture and we don't do it... when we eat in restaurants we pay the "coperto" (usually an euro or two) for the service of the waiters and you can't choose, you have to do it.

If I had to sell something I'd rather put it at a very low price than rely on tipping.