I think you can say that you are considering and will come back later. Like' Can you hold this for me one day? I will get back to you one day later'. Then the seller will be very happy to do so. In Chinese, it would be 能帮我把这个留一天吗？我一天后跟你联系。

可以帮我把这件东西留一天吗？（我会尽快来买的）

if you very want to buy this item,you could say like this"你好，我真的非常想要买这个东西，能帮我留一天，我明天来买吗？" if he or she say yes,you can say "非常感谢你！" you may try this way,but it not mandarin.good luck!

你可以帮我（把这个东西）保留一天吗？