The best answer is pricing.
Pricing is when a seller decides to release his or her products to the market and put a price on it to be able to be sold. In selling a product, a price for every individual item is needed to gain money from it from buyers.
If, for example, a person let his or her items in the marketplace without a price tag, people will think that those are not for sale. Some may even take these products thinking they are free or could speak to the owner if the products are open for a trade.
By pricing the items, you let the people especially the buyers know that they are for sale and you as a seller wants them to be paid with currency.