I've only seen two episodes of The Big Bang Theory, but if I were into writing specs and thought I had a grasp on the show's structure, I have an idea for the A plot:
Penny gets a box from her parents that was delivered to Sheldon and Leonard's apartment because she wasn't home; Sheldon, against his better judgment, signed for the box. She opens it there and it's full of her old Barbie stuff. Penny reminices a bit and Sheldon is horrified that she actually opened and played with her toys. He pulls out a Wonder Woman, mint in box, Mego. When she wants to look at it he "explains" his is worth something and hers is worth nothing. Thus begins their dark alliance into the seedy world of Barbie and Mego collecting culminating in a bidding war over the ultra rare black/asian 1972 Supergirl Barbie and the success of one and the defeat of the the other and the gloating that follows.
'Course that little paragraph is all it's ever going to be.