This method of legally obtaining free Robux will involve some skills and talents acquired based on your experience and time as a Roblox player, which, if you have them, you can choose one or both of the alternatives listed below:
Option 1: Selling your Creations in the Builders Club
If you have design skills for Roblox items, such as shirts, pants, hats, wings, among others, you can take advantage of it and put your creations for sale in the Builders Club market, along with the other items that you have purchased to other users, which will allow you to get Robux free of your sales.
In addition to this, you may also have the opportunity to offer your services to other users for script development by applying in the Group Recruitment Plaza section, formerly known as Group Recruitment Center.
If your construction skills are good, you can be hired by other users to build them some structure where you can get up to 2000 Robux for doing this job.
Option 2: Develop a popular game in Roblox
Another way in which you can get Robux for free legally is to test your skills as a developer by creating a game for Roblox.
Thanks to this, if you have enough creativity to capture the interest of other players to try your game, you can get a large amount of Robux through the sale of game passes, of which, you will get a commission of 10% if you are not registered as a member of the Builders Club and 70% if you have a membership in this platform.