Does Roblox TOS allow one to make a plugin on their own personal website, which users create an account on and pay real money through the website, to enable access to a plugin that will be used in Roblox Studio? If so, I’d appreciate any examples of plugins that do this. If not, where exactly in the TOS does it prohibit this act? Thanks. 
This is somewhat against TOS to sell Roblox assets outside of Roblox, but it’s still not that dangerous regards from this post.
If you feel it’s too risky, you can sell the plugin with Robux on the marketplace, but the choose is yours. Also, they probably can’t figure out your username if you sell it on your website