i just tried messing around with d&d's fancy426 code. i find that if you enter it as a guest it doesnt work. but if you use it on an account that hasnt used it before then it works fine. i believe they disabled this code for guest accounts in order to prevent people from using it more than once. ellusionist doesnt have this so you can use their codes as many times as you want on guest accounts but d&d won't let you. every time you use that code you need a new account.
i know it still works cuz i just tried applying it to an order on my account, but when i try it after i log out it doesn't work. it makes sense that they did this cuz i know ellusionist's codes get used by people more than once.
This makes perfect sense - I'm surprised that the other retailers haven't taken action to prevent this, since if you want to get technical, it's abusing the coupon code. I'm sure they all meant them to be used only once, since your registered account will only allow it once.
So now we can notate it: "single use only, invalid for guest logins".
Lushbob, the Seasons code is valid. It will work when the Seasons Deck website goes live, and will only be while supplies last - in other words, it's for the Seasons deck only, no other merchandise (assuming there will be any other merchandise). That image I got by "liking" Seasons Playing Cards on Facebook; it's straight from the source.