What an aggressive comment for no reason… the source is the terms of service, and every precedent established by Roblox themselves.
Roblox allows developers to sell assets that belong to them, i.e. scripts, models, etc because it falls under intellectual property, and Roblox doesn’t actually own anything a developer creates.
Groups are another story. Owners do not actually own Roblox groups, and therefore can not sell them for any sort of currency, including Robux. This is further supported by Roblox changing the wording on groups from “Owned by X” to just “By X”