Not only is it not a sentence, it is wrong.
Is the following sentence correct and natural?
Better than buy a rubbish gaming mouse that need to be nearly USD159
Not a teacher
Not only is it not a sentence, it is wrong.
I am not a teacher or a native speaker.
It's a absolutely simple sentence, what's wrong?
Not a teacher
It has no main verb, so it's not a sentence. It's also ungrammatical as a phrase.
For starters, it has no main verb, as Rover told you.
Typoman - writer of rongs
It has no punctuation. The verb that is present has agreement errors. At best it's a sentence fragment. It's incomprehensible to the point I'm not even sure what you're trying to say.
Wear short sleeves! Support your right to bare arms!
Even though you think it's simple (and you think it's correct), I think you should realise by now that unfortunately you're wrong. Not only have all the native speakers here told you it's wrong, we've also told you we have no idea what you're trying to say. It's both wrong and incomprehensible. The fact that you know what you're trying to say doesn't change that.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.