Is one who buys merchandise on credit, then buys it at a 7% discount, sinful, knowing that the seller is compelled to sell it at a loss due to his dire need for liquidity?

There is no religious impediment to buying goods for less or more than their usual price, if both parties agree and there is no other impediment. If you can lend the needy seller a goodly loan (qard hasan) so that he may sell his goods at their usual price, that would be better.