Someone told me that if I use my own ISBN on books I publish through Createspace that I would not be allowed to purchase copies of my own book. Though I think this is ludicrous, I would like to know, with absolute certainty, whether or not it is true. (I always use the free ISBN; it just makes my life a lot easier and I can't afford to purchase them.)
I have ordered many copies of my own books through Create Space, some of which have my own ISBN and others have a CS ISBN.
If you mean ordering books from Amazon per se rather than Create Space, I don't think I've ever done that, but why would you want to? If you order through Create Space, you get author pricing, which is much lower than the list price, even after subtracting the royalty you would get back. I'd be quite surprised if they stopped you from giving them money, though.
I don't know if the person who told you this was simply confused or if they didn't mean what you thought they meant or what.
I think the only differences between using a CS ISBN and your own ISBN on CS are: (a) if you use your own ISBN you have to buy it from Bowkers (or if you live outside the U.S., from your country's authorized source); (b) with your own ISBN you can't sell through CS's "library" channel"; (c) with your own ISBN your name shows up as the publisher, with CS's they show up as the publisher.
You can definitely buy your own books from CreateSpace, even when you choose the free ISBN. Here's a gratuitous self promo of my book, Do Nothing, Get Rich : You're Never Too Lazy To Be Rich (amazon link), which has a CreateSpace provided ISBN and I have ordered author's copies which I can give away, etc.
Copies which you order from your own createspace account are quite a bit cheaper also. The one issue you will notice is that he CreateSpace ISBN is registered to CreateSpace so your books show up as published by CreateSpace on amazon.com but not a big deal. My book shows this:
Publisher: CreateSpace Independent Publishing Platform (July 31, 2013)
So, in summary, getting the CreateSpace provided, free ISBN doesn't cause any problems at all. I really like the CreateSpace platform and it is linked directly to amazon's kindle (digital) books offering too. My book is available as a kindle book also. And when your readers buy your hard copy you can provide the kindle book for free as I do.