Most banks do not accept gold bars from private sellers, with rare exceptions in some private banks. This is a result of the rise in counterfeit gold products in recent years, plus banks not being able to evaluate gold bars efficiently to minimise risk.
Instead, gold bars are bought and sold on the commodities market. The most efficient and secure way for private investors to buy and sell gold is through specialist bullion platforms that hold gold in LBMA-member vaults, where price spreads (the difference between buying and selling prices) are the tightest and commission fees are lowest.