Buying gems in Guild Wars 2 couldn't be easier.
Log in to game
Press O to open the Black Lion Trading Company
Click the blue button that says "Buy Gems"
Select quantity of gems.
Enter payment information
Click Next.
Click Next again to confirm.
Enjoy the non-specific error message "Unfortunately, an error has occurred. Please try again later."
Repeat steps 4-8 about four or five more times.
Scream at your computer.
Open a support ticket with the details.
Wait an unspecified amount of time.
Be told to try again.
Cross fingers.
If it works, success. If it fails, go to step 11.
In all seriousness though, what the heck is up with the company that processes your payment information for in-game purchases? It's like a roll of the dice to see if it is going to work or not on any given day. I use one credit card for all my online purchases and I have zero problems with it anywhere EXCEPT when trying to buy gems from ArenaNet. I love this game so much, but this aspect reflects very poorly to me and shades an otherwise wonderful product with an incredibly frustrating customer experience.