Here's the deal ... Rob K. has established himself as one of the top dogs in the high-end sports card market -- often buying boxes of pricy, rare cards in the hopes of hitting big.

AND HE KEEPS HITTING BIG!!!

This time, Rob paid $750 for a chance at the contents of the BullPen BullChest at the Bullpen card shop in L.A.

Basically, 6 people pay $750 each to split the contents of the BullChest which is loaded with expensive PSA graded cards, boxes of unopened cards ... sometimes just packs of rare cards.

The collectors get to select 1 item from the chest in an order that's randomly selected by the shop.

This time, Rob was 5th to select (not the best position) -- but it worked out for him this time!

Rob decided to pick an unopened box of 2020 Topps Dynasty cards -- which usually runs around $500 and only contains ONE CARD!!

But, when the Bullpen staffer opened the pack (Rob was watching virtually on a live stream) ... he went CRAZY when he realized he pulled a special 1-of-1 Honus Wagner auto!

Wagner died in 1955 -- but the people at Topps secured an authenticated autograph used it as the base of a new card.

Unclear what it's worth ... but it's fair to say Rob EASILY made back his investment!

As we previously reported, Rob recently pulled an ultra-rare Tom Brady card that some believe could sell for more than $200k!