But how do you pay for a digital item and be guaranteed that someone else won't just copy pasta this item for free?
The only thing I can think of would be files with DRM protection. And in that case you'll usually be able to use the file only on one device.
I am not very familiar with the way DRM is made, but maybe an enhanced DRM-like protection can be made to the file is available only on the devices enabled by signing a message with the payer's BTC address, for example. (As I said, I am not familiar with DRM and I may have said something stupid).