The Deal with Magpie Decks
This is simple: It's a Tarot or oracle deck with cards taken from other decks. It may have exactly one of each card from a traditional Tarot deck - or not. I got my first unique magpie deck through Etsy and the seller included three Two of Cups cards - male/male, male/female, and female/female - because I told them I'm polyamorous and bi, and the Two oF Cups always comes up for important romantic relationships for me.
The Alleyman's Tarot is a well-known mass produced deck that was created to look like a handmade magpie deck. I'm not affiliated with its creator but the character of the Alleyman plays into the back story I mention on my About page.
If you're a reader, a magpie deck is an opportunity to remake the Tarot in whatever way you like, or create an oracle deck that suits you perfectly. You can have seven different Hermits and give them each their own special connotation, make a deck with only inanimate objects or only animals (I've done the latter), or anything else you can think of. Places to trade cards have popped up online, there are "booster packs" of a few cards available on The Game Crafter and Ifrom time to time) Kickstarter, and you can collect and trade cards for any kind of deck you want.
As a querent, it may just look like a novelty, but there's still the fact that your reader built the deck according to a plan. Decks have their own personalities and in this case, the choice of cards matters as much as an artist's vision does in a standard deck.