I found this weird premium tech tree bug that allows you to buy premium tanks that are not for sale for gold. If you go to the premium tech tree and go to the filters tab, you can choose any vehicle and purchase it via the drop down menu. It pops up the confirm purchase message or the not enough gold message. I do not know if this works or just a visual bug and I’m not gonna try to buy one to test it.
#Non-Purchasable Premium Tanks are buyable via gold.
lusty crypt
gleaming pewter
Tested it for you, "Item not currently in shop. Use a voucher or try again later."