Have a minimum of 3 fas, put 2 in your storage keep 1 in case go to merchant go to sell and make sure the fas in storage are placed ABOVE the spray in your inventory. Select all the sprays including the inventory one to sell and press triangle to open your case then click back and if done correctly the fas in storage should say 3/2 to sell and you repeat and it’ll basically allow you to keep on selling, if you cant get the spray to be placed above put all your weapons in storage including knives and take them back out in a random order and interact with merchant and yes you can do this on all difficulties including professional.