Are you sure you want to be selling something with a CC license? Technically someone could buy it and then post the whole thing on OGA. It would probably be better to come up with your own licensing rules that require the posting of significant edits but don't allow reposting of unedited material.
There have been a few people mentioning this to me, and part of me does worry. While my goals align fairly well with CC, I think that if this asset set ends up widely available for free to the public, I'll probably change it. I hope I figure out the licensing issues before I release another asset set.
And thanks for the concern :)
Actually I'm considering buying this assets pack because it comes under a free culture license, which means that I could use it to make a fully free and open source game. It is true that the license allows a buyer to share the artwork for free, thus bypassing the commercial entry point here, but that's kind of the point of a free license. The Attribution clause should at least ensure that people who may redistribute it would link to this itch.io page, though users who can afford it might consider supporting the original author directly.
The Share Alike clause even means that if people modify these assets, they should share them under a similar license themselves, which most would likely do free of charge. In practice, though, I've rarely seen people bother about it and publish their games using modified CC-BY-SA artwork without putting said artwork easily accessible online -- and most artists seem fine with it. The real "use" of SA would be more to prevent people forking the artwork and selling it as proprietary, which then would be a clear breach of license.
TL;DR: Thanks for giving commercial artwork + free license a chance! That's a great, yet hardly used, combination, and it does include some "risk", but I hope it turns out well for you.