Yes you can get banned for feedback manipulation. They know what you're trying to do. No one buys eBooks because they actually want them. And if they do, it wouldn't be 10+.
There really isn't a need for an account to have buyer feedback. Seller feedback is what you should be aiming for. If you absolutely want buyer feedback for some reason, you should only buy a few items, and stay away from obvious feedback builders like eBooks. Those make your account an easy target.
If you have access to the address on file, just buy some items that you actually need like shipping supplies, from sellers who always leave feedback and then you'd be solving two problems at once.