Is selling or buying in-game items legal or you will not get ban from roblox.
Below is a part of the terms of agreement that I have read through and found the part relating to this topic.
Paying for Random Virtual Items. If you provide games/experiences that allow users to pay (with Robux or otherwise) for random virtual items, you must let users know the odds of receiving each type of random item (that a user might receive) prior to paying. Please note that you must disclose those odds whether the user pays directly or indirectly. Here is an example of a user paying indirectly: Your game allows a user to receive a virtual coin in exchange for Robux, the user then goes to a different place in the game and throws the coin into a fountain, and then the user gets a random virtual item. In this example, you must disclose the odds of receiving each type of random item before the user throws the virtual coin into the fountain.
If you want to check out the whole terms of agreement here is the link