What kinds of items are the Gilded Playing Card Deck and Mismatched Playing Card Deck? Are they by chance playing cards that can be added to our collections?

they are toys :) you can look here and click on them to find out a trading card would say "Uses: Trading Card"

Trading cards go into the collections (most have Trading Card or Mahar Card in their name), the decks of cards are just toys.

Ah ha! All these years I've used the Item Directory Building and I never realized that clicking on the items was of any use, lol.

item directory could be improved some, but it is very useful if you know what you're doing there xD

Honestly, I use it all the time to make sure I haven't missed items for my wl. Didn't clicking on the items used to produce graphs or something like that?

it might have but not in a while as far as I know. I just use it to stalk new items when I expect things to get released or check which new items are books

Ordinarily I use Subetalodge for item info if I can't figure it out myself, but it's great to know that there is a more direct resource. :)