In many cases, I ask a seller about the price of an article, but finally I don't want to buy. I don't know how to reply him or her politely in English? What English-speaker usually say thin these cases?
From the link in J.R.'s comment above, the most common response is, "just looking."
No thanks, I'm just looking.
This is probably one of the more useful phrases to learn in any language.
Also mentioned is the related term "window shopping" but this is a general activity and not something you would usually say to a salesperson. "Window shopping" implies that you are just looking at the items for sale, and you don't intend to buy anything.
My friends and I are going to go down to the mall to do some window shopping.
Of course, there's no reason you can't be blunt, especially if the salesperson is pushy:
Wow! That's expensive.
Really? That much?
I'm sorry, I don't think so.
or my personal favorite
Hm. Let me check the price on Amazon ...