You can use Arnie’s discounts to make some credits, once you complete his missions. The best suited tanks to buy at a discount, and sell afterwards, are as follows (the list did not change since last year):
• M14 / 41 (Italy, MT-2): 2050 credits profit
• M15 / 42 (Italy, MT-3): 22700 credits profit
• B1 (France, HT-4): 92500 credits profit
• T1 Heavy Tank (USA, HT-5): 130500 credits profit. This is also the best choice because you can buy it and start accumulating experience for the future Yoh tech tree branch.
• O-I (Japan, HT-6): 95000 credits profit.