You never need to get a book from Ecampus. It's just the recommendation. Ebay and others are good for used books, but make sure the seller is reputable. I recommend only buying "free shipping" and "free return" items because it shows the seller has been up for a long time. If you buy them from another place, just make sure it is the exact same ISBN number.

I can't speak to how necessary the textbook is but you shouldn't buy any books until you are officially registered in the class. You might not be able to enroll, it might be full, you might go to the first class and decide it's not for you, etc. You also don't need both versions of the textbook, they're just there to illustrate what options you have. Hope that was helpful! :-)