Some relevant questions are whether the demand is elastic and how difficult to compare and/or fungible the product is. Medical care is inelastic (without it you die) and difficult to compare (you don't get to try the same surgery with two different providers). Food is inelastic (even more so), and easy to compare, but many foods are very fungible (your supermarket will let you choose red white or yellow onions, but not which farm your onions come from). Demand for clothing is more elastic (you need it but you can probably wait for a sale), and it's easy to compare the items.