Is it permissible to buy and sell from a person who buys things at prices much higher than their market value, then sells them for much less than their purchase price, and is it permissible to sell him mobile phone cards, cars, or anything else at the price he pays?

If buying by installment and selling for cash to obtain money (Tawarruq) is involved, then it is permissible according to the preponderant opinion. However, if buying for cash and selling for cash is involved, and the intention is to facilitate matters for people without harming oneself, then there is no objection to it. But if he does not have this intention, he is a spendthrift who must be interdicted, and it is not permissible to engage in transactions with him because that is a form of cooperation in extravagance and wastefulness. Scholars have differed regarding the nullification of a spendthrift's actions before interdiction, and the preponderant opinion is that they should be nullified.