I'm not sure if you mean having a business license? That would depend on where you live, most likely. As for needing a license or some kind of certification to prove that you're qualified to scrap-for-hire, nah....

all you have to do there is show samples of what you can do. The proof is in your finished work, really. Show your stuff!
On some store sites, such as here at the SBB Digital Bytes store, you can find the Designers' Terms of Use files posted before you go shopping, to see if commercial use is allowed, to what extent, and so forth. Sometimes that option isn't available to you, and you have to email the designer personally and tell them what your plans are, and wait for them to respond with their permission.
Commercial use means different things to different designers. For example, I don't allow anyone to redistribute or sell my files (and I don't think any other designers do either) or claim them as their own graphics. I also don't allow production of items such as clothing, etc. with my files, because I create my own clothing designs with them, and I don't want to have to compete with someone who is selling my own designs. I do, however, allow customers to use my products if they scrap-for-hire, as long as they are not a commercial outfit.
I know that the "little guy" might not have time or interest in creating lots of elements and backgrounds, and they just love scrapping. I like the idea of them using my stuff for creating scrapbooks for a few people locally.

I wouldn't want a printing company thinking it was okay to create scrapbooks with my files, and start mass-producing them, because that gets outside of the purpose I made them for. I spend tons more time designing than I ever do creating layouts. (Probably because I don't have kids, and my picture options are limited.) So it only stands to reason to me, that there are people who'd rather scrap than design, right?