Can you legitimately get robux from Roblox Games?

According to the Official FAQ, there are 4 ways to get Robux:

1. You can purchase Robux in our mobile, browser, and Xbox One apps
2. Accounts with a membership receive a Robux stipend
3. Accounts with a membership can sell shirts, pants and place access and get a percentage of the profit
4. Any user can sell game passes for Robux.

It further clarifies:

Question: Can I earn Free Robux?
Answer: No. Robux are purchased for real world currency and are only sold by the Roblox company

As for places that claim otherwise, see here

What should I do if I see scams?
Report messages, links, and games right away by using the Report Abuse links and buttons located around the apps and in every game menu. Our moderators will be able to take correct action against people trying to scam your account. You will also be helping to make Roblox an even better place!

You can adjust your Settings to prevent others from sending these sorts of messages by selecting Privacy and choose who is allowed to send you messages, chats, and more.