This story is ironic in a metatextual way, as in a few years Captain Marvel (and his family) would really go to Limbo, when the Fawcett comics closed up shop. There they would chill in the timeless void of fictional characters without a home, until the same company that had caused their titles to be dropped would buy up rights to them and bring them back as secondary characters. (Cap never showed it on the page but he must have been bitter.
Be that as it may, this is a typically charming little tale. Captain Marvel discovers that recent thefts of valuables are the work of the Collector, a wizened old coot who is assigned to take worthless inventions, badly written books, crummy art and such away. (From what I see on sale everyday, he hasn't kept up.) Limbo is a gigantic junkyard. The Collector decides to start confiscating something nice once in a while, just to brighten up his metaphysical realm. This is where Cap takes notice.
What an evocative splash panel. Wouldn't you like to spend a weekend or two rummaging around in the piles of debris in Limbo? Just keep that one door closed.
And (not for the first time) the World's Mightiest Mortal is stymied. Not even he can remember what has gone into Limbo, it's the very definition of the place. Back on Earth, the owners of the stolen items don't know what he's talking about. (Cap is suffering from something that occurs to all of us, where we rush into a room to get something, can't remember what it was and have to retrace our steps. This is called "Destinesia.")
Now, I love this next bit. The Collector asks Captain Marvel just how he could remember those items which were stolen. Nine times out of ten, a super-hero will obligingly explain everything, but Cap is shrewder. "Let that be MY secret!" he answers. Why tell the bad guy where they slipped up? So they can commit the crimes again without making that mistake? (As it happens, Cap figured there was a brief period between the thefts and when the Collector actually arrived back in Limbo-- and in that time, the robberies were written down in the police blotter, which he read. What a guy.