Wilt thou believe him? - That is, wilt thou trust him with the productions of the field? The idea is, that he was an untamed and unsubdued animal. He could not be governed, like the camel or the ox. If the sheaves of the harvest were laid on him, there would be no certainty that he would convey them where the farmer wished them.
And gather it into thy barn? - Or, rather, "to thy threshing-floor," for so the word used here (גרן gôren) means. It was not common to gather a harvest into a barn, but it was usually collected on a hard-trod place and there threshed and winnowed. For the use of the word, see Ruth 3:2; Judges 6:37; Numbers 18:30; Isaiah 21:10.

That he will bring home thy seed - Thou canst make no domestic nor agricultural use of him.

Wilt thou believe him that he will bring home thy seed?.... Draw in the cart, and bring home the ripe sheaves of corn, as the tame ox does? no; thou knowest him too well to believe he will bring it home in safety;
and gather it into thy barn; to be trodden out, which used to be done by oxen in those times: if therefore Job could not manage such unruly creatures as the wild ass and the wild ox, and make them serviceable to him, how unfit must he be to govern the world, or to direct in the affairs of Providence?

believe--trust.
seed--produce (1-Samuel 8:15).
into thy barn--rather, "gather (the contents of) thy threshing-floor" [MAURER]; the corn threshed on it.