I'll say it....
most "rookie" cards.
Could you imagine what you could obtain if you sold a NRMT 52T Mantle? Heck, you could get most of the T206 set and lots of other stuff (ok, maybe not NRMT, but....).
As mentioned before, not even his "rookie" card....
If I had a NRMT 52T Mantle I'd sell it and buy another lesser grade 52T Mantle card and a T206 Wagner in not so bad/great shape. Edited to add - oh yeah, and a 51B Mantle "rookie".