Items that you sell in the shop are counted towards this achievement based on the number of items that are sold in a bin at your shop. Each bin can hold a minimum of 1 to a maximum count of 10 of a specific item, so if you fill a bin with 9 bricks and it sells, this will count as 9 towards the total for this achievement. The 250 items that you sold to complete the bronze and silver medals on the way to this achievement will also count towards your 4000 total. Any item in your warehouse is eligible to sell in the shop. you can co-ordinate with other players that are friends to buy and sell batches of items to each other to accelerate the completion of this achievement