not true. from my personal experience. If you buy reputed coin/bar (credit suise or maple or krugrrand ) they are 24k (or of equivalent weight, i.e. Kruggrand may be 22Ct but but it's weight is more then an ounce to make up for added copper). When buying you do have to pay mark up (varies 2-5%) but selling it's prices as quoted.
and unlike jewellery, you can sell in any shops (who is giving right price)