Does the sin fall upon the buyer or the non-Muslim seller if the buyer is harmed by a penis enlargement medication purchased online, given a previous fatwa that permits it as long as it does not cause health harm, and the sellers assured its harmlessness?

If it is scientifically proven that the medicine causes no harm, then there is no legal impediment to its use, so long as it does not lead to a religiously forbidden act. This must be confirmed by consulting specialists. There is no sin upon you if the selling entity is accredited and you trusted it; the sin falls upon them if they commit fraud.