@mariokluser is right. Do your research. Hire a seller you feel you can trust. I’ve had a few buyers ask – even require – me to sign an NDA before working with them. I don’t sign NDAs here on Fiverr, partly because that often requires communication outside of Fiverr (such as with a signed document), or the exchange of personal information. Both are against the rules of Fiverr.
A few of those buyers chose to not to work with me, because they, personally, NEEDED my signature. But, most of those NDA buyers actually chose to work with me despite the lack of an NDA. I have proven my trust to those I work with, and they were ultimately satisfied with my promises and guarantees (which I am still keeping to this day).
There are other sellers like me here on Fiverr, but you’re going to have to research to find them. Ask around, communicate with potential sellers, gauge their trustworthiness. This will require work on your end, and ultimately, it’s your faith on the line. Us good sellers are out there. You just have to be willing to find us.