Hey there,
So if you make a design you are the artist of that design. But when you sell that design to somebody you no longer have the right to use them unless you have permission for that. Reason is, if you upload your delivered work to Fiverr Gig without their permission Fiverr wouldn’t mind. Since those are your works. Buy Fiverr wouldn’t mind untill one of those clients files a complain to Fiverr. And boom you may straight up loose your account and also get a lifetime bann on Fiverr.
So yes, You can use your works but first you have to ask for permission from your clients and better to have them in some kind of written agreement. Or atleast ask them in Fiverr message if they are your clients from Fiverr.