Type /shop price Niobium #(this is the number of Niobium as indicated on your shop page) #(this is the price of the item) 0(the min stock your base allows before it stops selling) 100000 (the max stock your base will allow before it stops buying).
So if Niobium is number 4 and you want to sell it for 4000 per unit it would look like this.
/shop price 04 4000 0 100000
That should work. It works for me anyways. I can buy platinum and don't have to log in as an admin.