Whenever I press the "Buy With Paypal" button to buy an item, it pops up with a button that says "Confirm Purchase". When I press that button, it goes away, and nothing happens. Is it because a Paypal account isn't linked to my account?
Are you trying to buy via forum shop or in-game one? If it's game client, make sure you're on the newest version (5.43) - when attempting to buy items with PayPal it should bring up paypal auth page in your browser. Adblock tertywerty mentioned above should be irrelevant, it doesn't affect authentication in any way.
All you should see when attempting to buy the item is the screen that says
Please wait, your order is being processed and you will be redirected to the paypal website.
If you are not automatically redirected to paypal within 5 seconds...
[Click Here]
and then be brought to the PayPal authentication page. If you're getting something else, please post it here - I just tried purchasing an item to see if there's anything that indeed broke on our end and it worked fine.