if I receive benefits from a country because of an illness, is it allowed for me to buy [for business purposes] products at the wholesale or retail price and sell it to make some money on the side [extra money]?

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

If you mean that you wish to invest the benefits or part of them in trade, then if this money is given to you so that you would dispose of it freely, there is no religious impediment to investing it in trade and gaining profit from it. However, if the country that gives you the money stipulates spending it on a specific purpose, then you may not go against this stipulation.

The Shaafi‘i book Asna Al-Mataalib reads pertaining to the situation in which it is stipulated that the offered money be spent on a specific purpose:

"If he gives him some dirhams and says to him, 'Go buy yourself a turban' or 'go to a public bath' or the like, then it is incumbent on the person who receives the money to spend it for that specified purpose. This is the case if the giver intended by specifying the purpose that he covers his head or cleans his body because he saw him in need of these things. However, if he mentioned these actions as mere examples, as is customarily understood, then the receiver of the money is not obliged to spend it on those purposes; rather, the money becomes his personal property and he is entitled to dispose of it freely as he wishes."

Allah knows best.