As with all tradable assets, the buying and selling price of gold are different because of the profit motive. If you're using a coin store or gold bar retailer, the dealer's profit is the gap between the price they sell to you and the price at which they buy back.
In an open gold market such as BullionVault, the gap between prices to buy and to sell is known as a 'market spread'. You can access and change this spread directly yourself, bidding or offering gold at different prices to those quoted by other investors.
Track the changing ‘spread’ between the bid and offer prices on BullionVault’s live order board and see the actual prices being paid to buy and sell gold.