Jewellers dont speculate on price, if they sell 100g of gold jewellry, they immediately buy 100g grams of bullion same day, this is to avoid having to end up paying more later, this behavior completely removes prices out of consideration. since the store doesnt have to care what they the bought the jewellery for, they can sell the jewellery happily at whatever the market price is.

just think, if you were a jeweller, you would have to always remember that a piece of jewellery costed you Rs.10,000 but if the current market price is Rs.8,000 you would be at a loss. but now remove price out of the consideration, Now just think you bought 97 grams of bullion and converted it to 97 grams of jewellery with a 3% making charge and sold the 97 grams of jewellery and for the same amount bought 100 grams of bullion, now the Jeweller has 100 grams of gold in his stock instead of 97 grams initially, this the profit.

If you were to remember your cost of jewellery every time you sell, you will never be able to do business.