In The Witcher 3: Wild Hunt, you can buy and sell items and repair and disassemble them. However, the prices are based on the item's level and the costs for buying and selling ammunition and weapons.

Deleveled Gear Buying Selling Servicing Prices allows you to set the prices for buying and selling items and repairing and disassembling them, regardless of the item's level. This is done by adding a new row to the table that sets the prices for buying, selling, and servicing weapons and armor.

To do this, you need to install the mod, and then you need to open the table in the game. You will see the new row you need to add to the table.