Does selling enchanted furniture to someone I do not know make me responsible for their harm, or will the enchantment not harm them in this case?

If the doubt has no basis, then disregard it and maintain the prescribed remembrances (adhkar) and seeking refuge (ta'awwudhat). However, if you wish to dispose of the furniture, it is not permissible to sell it while concealing its condition if harm is feared from it, due to the Prophet's (peace be upon him) saying: "There should be neither harming nor reciprocating harm," and "Whoever cheats us is not of us." The seller is obligated to disclose any defects in the item. To ascertain whether harm would befall the buyer, please consult specialists in Ruqyah Shar'iyyah and magic.