Is it permissible for a bank to purchase a commodity, and then for me to authorize the bank to sell it to me for a 6.5% markup?

If the bank purchases the commodity and takes legitimate possession of it, then sells it to you, and does not buy it back for itself, and there is no contractual or customary obligation on the bank to sell it on your behalf, but rather it is at the option of both parties, then there is no harm in this transaction.