Building placement means absolutely nothing.

You stop gaining attack points when the item you buy doesn't give you any more points that the lowest item already in your inventory.

You can only take 500 mafia members to a fight, therefore you can only bring 500 of each item to a fight. So, for example, if you have 500 mafia and 500 guns and your lowest attack point gun is 2 points, you won't gain anything extra if you buy an item that has 1 point of attack. However, if you buy an item that has 3 attack points, then that 3 attack point item will replace your 2 attack point item and your total attack score will increase by 1. Does that make sense?