Is it permissible to purchase platinum with constructive possession, given that physical delivery of it from the bank is unavailable?

If the commodity is owned by the seller, it is permissible to sell it whether it is present or absent, provided it is described in a way that removes any ambiguity. Gold, silver, and currencies are exceptions, as they require simultaneous exchange (hand-to-hand transaction) in the same session. The mentioned transaction falls under the category of organized (bank-facilitated) Tawarruq, which is impermissible, because the buyer authorizes the bank to sell the commodity to obtain cash. Therefore, if you wish to buy platinum from the bank and sell it, you must sell it yourself and not authorize the bank. Since the bank does not provide a platinum delivery service, buying from it is impermissible.