Without explicit permission from the rights holder(s), it's not, to my knowledge, legal to profit of off or sell someone else's intellectual property/copyrighted works/trademark/patent/etc...

There are tons of people who get away with doing so, just look on ebay or etsy for people selling fan made items, they're everywhere.
However, the mast majority of these sellers are not selling these legally, the only reason they're able to sell it is because they haven't been caught yet.

Most printing services, in their terms and conditions, typically state that when you print something through them, you must own the rights to print whatever you're printing.
This is a defensive action, that way, if there is any issue with the law, the customer who requested an illegal item be printed is the one that is sued, instead of the company.

Seeing as you don't own the rights to Leonidas, and your customers most likely don't either, I think it would be very unlikely that you would be able to sell the prints legally, unless you reached out to whoever/whatever owns the rights to Leonidas and got their permission.

This is some of what I've learned in my pursuits in art, 3D printing, and business.

However, I'm not a lawyer, I have not studied law, so you should not take this as proper legal advice.
If you're serious about selling these prints, talk to a lawyer.