The seller may have started the car cold and shut if off right away. Sometimes if too much fuel gets in the cylinders it could make a puff of blue smoke come out the exhaust. I have seen this on cars and trucks before and it will not do it on every startup. If you are really serious about the car start it up again cold and see if the car puffs out blue smoke again. If it does, tell the seller and see if you can negotiate a discount to fix it after the purchase.