In the translation at the end of Chapter 9, Wheelock gives the definition of "capto, -are" as "hunt for legacies," but that doesn't make sense in the translation:
Habet Africanus miliens, tamen captat.
African has millions, nevertheless he hunts for legacies.
Capto, -are is a transative verb, so I assume it refers back to miliens.
Has millions of what? Millions of legacies? I get the sense the miliens means the same for Martial what it would for us today: millions of dollars.
I looked up several meanings for capto, -are. "try/long/aim for, desire; entice; hunt legacy; try to catch/grasp/seize/reach (Whitaker, Latin Words)", "to catch at eagerly; to keep reaching for; to try to catch, case after; to strive after, long for, desire earnestly (Traupmann, New College Latin Dictionary, New York: bantam, 1966)
"Africans has millions (of dollars), nevertheless he grasps/longs for/ strives for (more)."
I did a web search for Martial 12:10 and that does seem to be the meaning of the epigram.
Now, far be it from me to criticize Martial's writing ;). Shouldn't Wheelock given a better definition for capto,-are?