Is a car seller sinful if he bought a car and did not pay its price until after selling it to another party, while making a profit without the knowledge of both parties?

It is permissible to sell a commodity before its price is paid, provided that the purchasing process and taking possession of it are completed, due to the Prophet's (peace be upon him) prohibition of selling what one does not own and selling before taking possession.

As for informing the buyer of the price at which you bought the commodity: - If your informing of the price is truthful and you sold it for a higher price, there is no harm in that. - However, if you lied in informing of the price, then lying in sales is forbidden and eradicates blessing. You must repent and return the excess amount along with its share of profit to the buyer. This is the view held by the majority of scholars.