It sounds like what you are really doing is selling your _services_ to your client, where they are paying for your time to create a site for them. From that they would effectively keep the site you build for them to use as they see fit for the lifetime of the website (I wont go into the whole copyright side of things, but it's often implied that when you build a site for someone you don't give them the copyright to the code as such, but a 'perpetual licence' to use for the lifetime of the 'project').

So in that case it's really no different to use Cloneamental versus the built in Elemental theme to build a site, you're free to do that, including hacking away at it to suit a purpose.

In the case that you're making it clear that they are purchasing actual software, it's a case of recognising that concrete5, which include the Elemental theme is under an MIT licence, and therefore Clonemental has that licence as well.

MIT licences are very permissive in that you can actually sell MIT licensed software to others, even if you didn't write it yourself, the main requirement is just that you continue to include the MIT licence. If you are following these rules, you're fine.

It really then comes down to more of an ethical question to be honest. If you are just taking something that is under an MIT licence and free, and selling it to someone as is without adding extra value, that's legal but really not that honest. Whereas if you take something MIT and then add lots of stuff, hack away at it, make it have a new value because of your input and then you decide to sell it it's a bit different - customers are then paying for the extra value (and arguably moreso your services rather than a product), and your pricing would probably reflect the value of your work you've added rather than entire codebase.

Cloneamental is deliberately MIT licenses so you can do what you like with it. That is because it is nothing more (or less) than a packaged clone of Elemental, which is bundled with the core and also MIT.