In the age of the Internet, copy protection is a fool's errand, and it's never worth your time and effort to even try.
Consider Microsoft, who spends more resources on R&D in a single month than you'll ever have in your entire life put together. They're obsessive about copy protection, and yet the latest version of Windows had a crack available before RTM. If they can't pull it off, why would you ever think you have a chance?
The deck is stacked against you from the beginning. First, we have the theory. You can frame the basic issue here as the fundamental problem of cryptography: Alice wants to send a message to Bob, and ensure that Charlie is not able to read it even if it should fall into his hands. Except in this case, Bob and Charlie are the same person.
Now we have the details. If your system downloads something when a person gives the right account credentials, then a person with the right account credentials can download it. It's really that simple, and any additional complexity you introduce will cause unhappy customers.
A central server makes sure that you're logged in properly before the software will launch? Nice going, bozo. Now anyone with a flaky Internet connection can't use what they legitimately bought and paid for.
A server ensures the user isn't logged in twice at the same time? You just tossed a dozen very useful testing scenarios out the window.
Fingerprint the computer so they can't re-download to a different system? Sounds great, right up until that computer dies and they get a new one.
Bottom line: don't even try. It doesn't work. Never has, never will, never can. All you end up doing is wasting money, annoying your customers, and driving them away. There is one and only one way to make money in software, and that's basic market economics: Sell your product for less than what the prospective buyer sees as its value to him, and he will pay for it.