Why is there a difference between gold's buying and selling price?

Gold like any other tradable commodity has a buy-sell spread (difference between prices). This difference changes on the basis of various factors like price volatility, supply, external market conditions, etc. Even if you buy gold from an offline jeweller and plan to sell it after some time, you will get a lower price for it. This is a standard gold trade industry practice.

The buy-sell difference is ~5.7%. This includes 3% GST paid to the government on gold purchase and remaining charges for insurance and locker to safely store the gold.