What is the ruling on buying medicines from those who benefit from health insurance, given the difference in their intention and need to sell them?

It is permissible for someone who sells medicine to do so if they acquired it through legitimate means. However, if they obtained it through illegitimate means, then it is not permissible for them to sell it. Whatever is permissible to sell is permissible to buy, and whatever is not permissible to sell, is not permissible to buy. If the status of the medicine is unknown, the default is permissibility. All of this applies unless the responsible authority prohibits the sale or purchase of these subsidized or free medicines. If they prohibit it, then it is obligatory to comply with that.