Sometimes, you want your books faster, or think you can get faster shipping for a cheaper price, or want to try to “bump” your sales ranking using purchases from Amazon. I don't see any problem with that and am not aware of any conflicts with the terms of service of either Amazon or CreateSpace (although I haven't looked into them specifically for that information.) For shipping considerations, I considered doing that myself until I realized that I didn't really need to.
CreateSpace prints & ships from North Charleston, South Carolina so depending on where you live you'll probably get the books much earlier than they predict on their website, especially if you live in the Eastern half of the United States. You can plug in 29418 to UPS to see what their estimated ship times would be – you'll have to guess on the weight, but I know that when I buy 20 books it's usually about a 9 pound box. Living in Maine, I usually get the books in about 3-4 days, and when I order for a client in Tennessee, it takes 2-3 days, with standard shipping from CreateSpace.
If you are purchasing from Amazon and taking advantage of “free” 2 day shipping using Amazon Prime, then you have to figure that your 40% cut from the list price is what you are actually paying for shipping. The production costs remain the same in both cases. It could potentially save you quite a bit if you are ordering a lot of copies of your book and you need them right away (Amazon allows you to order up to 30 copies in one order) but in most cases it will still cost you more than ordering with standard shipping.
For example, one of my books has a product cost of $3.17 and the lowest I can price it at and have it listed at Amazon is $5.29 (which results in my not receiving a royalty.) Here are the costs with the different shipping methods that I have available:
- $2.12 (per copy) w/2 day delivery through Amazon Prime (any quantity)
- $0.46 (per copy) w/5 day delivery through CreateSpace-Standard (at 50 copies)
- $1.00 (per copy) w/2 day delivery through CreateSpace-Expedited (at 50 copies)
- $2.26 (per copy) w/1 day delivery through CreateSpace-Priority (at 50 copies)
- $2.12 (per copy) w/1 day delivery through CreateSpace-Priority (at 109 copies)
Unless I'm ordering 110 or more copies through CreateSpace and need guaranteed next day delivery, it doesn't make sense to use the Amazon trick. Remember that I can get the books in 3 or 4 days (including production) at standard shipping where I live, and obviously your books will be a different number of pages and potentially a different trim size, but in general it doesn't make sense to use Amazon Prime.
If you just want to pad your sales numbers, well, I wouldn't bother; it's an expensive way to do so, it will only last for a short period of time, and it's a little like gaming the system. Even if you won't be penalized now, it doesn't mean that they won't wise up to it in the future and ignore your purchase anyway, so I wouldn't use that as a reason for pulling the Amazon Prime trick.