Is it rude to not say hi someone if they say it to you first?

Yes it is.............

Sometimes yes, sometimes no. In any case, it's not worth being offended. I know it can be discouraging when it happens, but don't let it sour you on being friendly.

Yes, you could smile, give a wave, or just simply say hello.

Depends. You don't have to verbalize it...just job, smile or wave.

I started doing that since thats what people do when i say hi first