Is it permissible for me to profit from selling medicines that I purchased with my own money, then sell them to customers without informing them of the profit margin? And is the money I earned through this method unlawful (haram) and must be returned?

If people deal with you as their agent, and you do not profit from them, then it is not permissible for you to profit except with their knowledge. You must either seek their forgiveness or return the excess money to them by any possible means, without needing to inform them of the full details.