Is it permissible to trade in cars that a company put up for sale after repossessing them from owners who were unable to complete their payments, without returning what they had previously paid, even after the sale?

It is not permissible to deal with this company, which sells cars through a lease-to-own system, because it involves gharar (definition) (excessive uncertainty) and risk. The buyer might pay the installments and then become unable to pay the last installment, thus losing all that he has paid, while the company benefits from both the price and the item sold. This falls under the category of consuming people's wealth unjustly.