I buy silver, and every now and then gold, off one of the better known websites.
https://www.apmex.com/
Basically, if you are buying, you pay X number of dollars per ounce over the spot price. If you decide to sell, you sell for X number of dollars per ounce under the spot price. Of course, this means you are going to lose money unless the spot price increases enough over what you paid to show a profit.