Idk magic but i found 40k cards

I found a collection which i estimate to be 40k cards (4 boxes organized nicely-~35k- and 2 with cards thrown in)

Im considering bringing them to my local game store and i want to make sure i dont get a shafting when i bring them in. I dont have a clue about the value of anything related to cards.

Would you give me some advice, please?

EDIT: A lot of people are suggesting its "found". I run a junk removal company and they were left in a basement after a tennant was evicted. Not stolen (by me) and not interested in finding the old tennant.