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.