I've tried buying in a different browser, logging out and back in, logging out for some time before trying again, on a different computer.. I'm sure there was other stuff I tried but I can't remember. But I get that 5 second message no matter what I try.
I was legit RSing, and not even that much! I don't play too frequently/seriously and then all the sudden I go into a shop and that happened. Like I said, I haven't been able to buy anything for quite a while because of this. I know that it should only happen if I try to buy something after immediately purchasing something else, but that's not what has been going on. Guess I'll just keep on waiting. I just figured I'd ask to see if anything like this had happened to anyone else because I was and am pretty confused by it. Thanks to those that gave helpful replies.