He gets the product and sells it for his own benefit when the item is not available in the store where he works

It is not permissible for a worker or employee in a store
to sell for his own benefit, whether the product is available in the store
or not, because the time at work belongs to the owner of the store. So he
has no right to use that time for anything that does not serve the
interests of the owner, because according to business custom, this is
regarded as a betrayal and cheating the business.. That is because
usually, if the worker does not find the product in his store, he will buy
it from his neighbour and sell it to the customer for the benefit of the
store owner, not for his own benefit. 

Based on that, what you are doing is contrary to your
employment contract and it is cheating and betraying the store owner. 

Based on this, you have to refrain from doing this action and
repent to Allah, may He be exalted, from this haraam earning. You have to
work out how much money you acquired in this way and put it in the store,
because it belongs to the owner of the store and it does not belong to you. 

And Allah knows best.