It's users, plural. The verb has to agree. Users buy X. Users don't buy Y. Users who don't buy Z. Saying "users who doesn't buy something" would be ungrammatical.
Edit: I took the "something" in your example to be a placeholder for a product name. If that is not the case, i.e. if what you actually want to say is that there will be users who will buy nothing at all, then, as JoseK points out, you should be using "anything" rather than "something". That's because anything is a negative polarity item, while something is a positive one. So, to sum it up:
- There will be users who don't buy X.
- There will be users who don't buy anything.
- There will be users who buy nothing.
- * There will be users who doesn't buy something. (Ungrammatical.)
Both are correct. There are instances where they mean the same thing and some instances where they don't.
If you were telling someone that you didn't intend to go out tonight, you could use either.
"Do you want to go with us to a restaurant tonight?"
"No, I think I'll stay home."
"No, I think I'll stay at home."
However, if someone were asking where you were staying, where the answer might be "a hotel" or "a friend's house", you would definitely say "at home".
"Are you going to get a hotel room for the conference?"
"No, I'm going to stay at home."
"No, I'm going to stay home."
(Note that if you did say the struck-through version, that would tend to imply that you weren't going to go to the conference at all.)
However, that "home" is uncommon in that it also functions as an adverb. You can never leave the preposition out with other similar nouns. For example, this is clearly wrong:
"I'll probably stay work."
In that case, you have to create an adverbial prepositional phrase:
"I'll probably stay at work."
Best Answer
Both are grammatical expressions, and common.
This can refer to a TV program that will be broadcast this evening. Or it can refer to some other form of entertainment. Or it can mean there is a confrontation looming.
This means some prearranged plan or meeting is to take place tonight.
The "it" in both cases refers to some unspecified matter which will either be understood from context or explicitly stated before or after these sentences.
That brings us to the simple expression
This is the confrontation I referred to above (often given as "Oh, it's on!") meaning that the speaker has accepted or recognized that a challenge of some sort has been issued and means to accept it, usually with relish or special determination to win.