What is the ruling on selling goods that I do not own, and setting their price either as a certain percentage or as a fixed amount with the remainder for me, and does selling on credit affect this ruling?

It is permissible for the owner of the goods to leave them with you for sale in exchange for a commission, even if the commission is the excess over the price set by the owner, such as him saying: "Sell it for five hundred, and whatever exceeds that is yours." This is permissible according to some scholars. As for selling on credit, it is not valid unless the owner of the goods explicitly or customarily permits it, because an agent does not act except within the limits of his principal's permission. If he permits you to sell on credit, then there is no objection.