I have felt your pain and frustration!!!
Now over 6 years ago. I received over 180,000 cards from my brother (my nephew's collection). All sports and non-sports cards. Years from 1940 to 2012 not a lot of 1970 or older and 2003 and newer. Base, Parallel, complete sets, partial sets, Graded, Auto, Mem, SNed (a few real low one), pins, mins, oversized, oddball cards and stuff (packages of gum with baseball teams on them). Not a lot of one thing Just a lot of a little of this and a little of that. (yes, a box of pro Ser football or 88 baseball).
All the boxes were marked but I find out that were WRONG. He had them all messed up. I opened a 3200-count box and find (you name it. It was in the box). That was the way all the boxes were. A $100 Basketball card in with 1990 Pro set football cards. I now go through the sets marked complete to verify before selling.