@vickiespencer is correct. Fiverr’s Terms of Service state that all work, copyrights, etc. become the full property of the buyer upon delivery, unless you state alternate terms in your gig description (which would, in a practical sense, be extremely hard to enforce outside Fiverr to begin with). In other words, once you deliver, the buyer owns what you deliver.
I’ve never understood the value of “commercial rights”, as a purchasable element here on Fiverr, because you can’t control how the buyer is going to use your services. It’s not like we have written/signed contracts that can legally define the ongoing usage and licensing rights here on Fiverr, and then use those agreements in litigation if breached. We aren’t allowed to have those kinds of agreements (since they usually involve off-line communication and/or personal information/signatures).
The best way to combat things like this here on Fiverr, is to price all of that into the cost of your gig packages, and learn to be okay with the purchased work no longer being yours upon delivery.
Personally, I have many skills that are not featured as gigs, because I don’t want to give up control of that level of work, and not be able to enforce its protections outside of Fiverr. I only offer gigs for services that don’t need extensive legal protections (from my end) in the outside world.