Depending on who you ask, there's technically nothing wrong with "vote for vote".
However, in my experience a) The author hasn't read your post and they're automatically spamming 100s of posts (if that's the case, then feel free to downvote them) and b) even if it was legit, you're wasting your voting power that you could be using to upvote/thank people that are legitimately interested in your content.
In short: It's 100% 'legal' , but frowned upon by most, and probably a waste of your VP, so best to just ignore (and/or downvote).
P.S. Downvoting uses up your bandwidth and VP as well, so use sparingly in the beginning days.