What TABOB said. Here's an industry standard breakdown for working with freelance artists. I'd strongly recommend if you have questions on how the freelance art industry works, you grab a copy of the Graphic Artists Guild Guidelines.
Long story short, when you work with me as a designer, I do not ever sell you the design.
What your contract with me will clearly state is that I'm licensing the design to you, which means I'm giving you permission to use my intellectual property in a specific setting.That means, if I sell you a license to print an illustration on posters, you don't actually have the right to also use it for other products. So if you want to print it in magazines or on clothing, we need to write another contract and you'll need to pay for those rights. Even when I make art solely for your use, I won't sell you the rights. It says so explicitly in my contract that you're instead getting an exclusive license and that I'm retaining the rights and reserve the right to re-use elements of it for other things.
It's standard practice for designers and artists to retain the copyright for their work, which means if I ever turn around and sell the copyright for that artwork or another license to someone else, it's not theft and it's perfectly within my rights as the copyright holder to do so.
In most freelance art industries like illustration or apparel design it's actually factored into the costs of doing business. It's generally considered bad form to re-license a work without informing the previous client and giving them an opportunity to purchase the copyright to protect their interest and license, but almost all truly professional freelancers factor it into their business that they are going to re-use elements of prior art and modify it to speed up their process. Especially on Fiverr, if you dig through the artist's terms, you'll probably find that they charge extra for actual design files, and/or that you're just getting a license. A lot of times $5 on there just gets you a low-res .jpeg of the art that's not reproducible and you'll need to arrange terms with them for the full thing. Honestly, you probably won't find many of the really professional artists and designers on sites like Fiverr or Upwork, specifically because those sites have stock contracts that automatically take rights away from them like someone already mentioned.
Frankly, (and this is just my experience.) but any artist or designer who is willing to automatically sell you the rights to artwork without compensation for the loss of future revenue is almost certainly desperate or an amateur, and you're giving up getting good work in exchange for that. Really good, professional artists all generally go by some approximation of the Guild Guidelines. While most of use will negotiate on rights, you have to understand that if you want exclusive license or outright ownership of rights, you're going to pay "extra" for it, one way or the other.
