Dear when buying a product for sale, when I want to enter it through the tree accounts is not through the stock
Do I have to enter it twice, one of the revenues (for sales) and the other out of the expenses (as purchases)?
It appears that you are new to accounting, so it might be better to have a look at the tutorials linked to in this thread before posing questions Tutorial help
If you purchase something for sale, then until you sell it, it will be part of your inventory
If you are just a middleman and don’t hold stock, then you will have to enter two transactions - a purchase and a sale