Am I entitled to supplicate against a merchant who exploited my need for money and bought goods from me for much less than their worth, despite my final consent to the sale?

This type of sale is the sale of one in dire need, and its validity is a matter of dispute, with the majority affirming its validity, albeit with the strong disapproval of buying at such an exorbitant loss. Instead, one should assist the person in dire need rather than exploiting their necessity. The sale of one in dire need is of two types: either compelled to the contract unjustly, in which case the sale is void and does not take effect. Or compelled to sell due to debt or necessity, so he sells his property at a significant loss. This type of sale is valid, though strongly disliked. Allah Almighty has forbidden people from diminishing the value of others' belongings, saying: "And do not deprive people of their due." The sale is valid, and one should not supplicate against the buyer.