Depends on your employer, your contract, your domain, location, etc. If you signed a non compete agreement when you join your employer then you need to make sure your own business isn't related to the same domain, etc. Distribute your product for free if it is a problem, then when it’s time to quit your job, introduce a non-free version of your product.
In Russia it is totally legal, however employer wouldn't be happy, but who cares.
I was thinking for your employer. If you piss them off then it's the same situation as if you have quit.