Welcome, Philipp,
If you're looking for information about decks we have topics about already, you need not go any further than those topics. The Playing Card Plethora covers modern decks, defined as being 20 years old or less. A Cellar of Fine Vintages covers vintage decks (approximately between 20 and 90 years old, or made between the 1920s and 1996) and antique decks (pre-1920s). If you have a deck we don't have a topic about, go right ahead and start one - you might find that someone else out there has the information you're looking for but never got around to posting it until you asked!
The forum owner, the late Tom Dawson, co-wrote a book with his wife, Judy, called "Hochman's Encyclopedia of American Playing Cards," which covers decks made prior to 1950 and a small number of newer decks. The book, and its related Price Guide, recently came out in an all-digital second edition, available from the Conjuring Arts Research Center (conjuringarts.org). That would be a useful guide to help you with pricing some of your older decks. For newer decks, I often check eBay, but I look only at completed sales, not what people are asking for their decks - a true guide to a deck's value is what someone's actually willing to pay for it, right? But even there, you need to keep certain factors in mind - for example, if a limited-release deck suddenly hits the market, the market tends to get flooded with them, driving prices a bit lower because of the high number of decks offered at the same time, whereas if a future release had an advance sale of very limited numbers, you'll see few decks going for high prices - but depending on the deck, it could become a wider release in a short time, making the price/value plummet.
A good resource for playing card knowledge would be the collectors' clubs dedicated to card collecting. This forum is the official forum of 52 Plus Joker (52plusjoker.org), a club based in North America but with an international membership. Among the many member benefits is a members-only section of this forum, two club publications (a printed quarterly and a digital monthly) and an annual convention - the next one will be in October in a suburb of Chicago, Illinois, USA. We're also friendly with a European-based international club called the International Playing Card Society (i-p-c-s.org) that might be right up your alley.
Please, make yourself at home, feel free to ask questions, and above all, have a good time!