I’d say it’s actually uncommon not to release full copyright to the buyer once they have purchased the order. I’m not a tee-shirt designer, admittedly, although I have looked at a few gigs for a possible future project, but with content writing, once you’ve given something to the person who bought it, you no longer own it. I couldn’t rock up and tell one of my clients at some point in the future that I want to stop them from publishing the work I had sent them, because they’re turn around and be like, “Well, that’s tough luck” and rightly so. In short, once you’ve received the shirt, it’s yours to do what you want with it. It might be a good idea to message the seller just in case, but I’d say you would be fine to order under the pretence of once you’ve bought it, it’s yours.
If you can’t find someone who shows that they can do silhouette images on tee-shirt designs, it might be worth making use of the ‘buyer request’ feature. This way, you can ask that people send you samples of their work and set a budget for the product. You’re more likely to get several responses which each have a different level of talent so that you can go through them and choose the most talented option for your budget. You could request that they complete a sample at a reduced price based on your drafted version, but you should understand that if you go down this route, not every seller --especially more established ones-- is going to feel comfortable or able to do this for you.
You look for somewhere where they offer a ‘getty image’ extra. For every extra $10 you pay, you get a free image. As I said, I’m not in the design area, so I don’t know for sure whether you can only buy one or several with each gig. That might be worth looking into.
Otherwise, you can look at the reviews that people have already received from other buyers and take note of what others are saying. If people are generally reporting good things about the seller, then you should be on the safe side. However, remember that if you do happen to hire a seller that does steal an image, your money is held by Fiverr and can be refunded so you aren’t losing out for their breach of copyright. You might want to do a reverse google image search with the orders before accepting it to make sure it truly is original.
Obviously I’m not as much help as someone who is in the design area on this website, but I thought I’d try my hand at giving you some beneficial advice anyway based on what I do know about the website. I hope you manage to find a buyer who can help you produce high quality tee-shirt designs soon.