 Why did the odd beings from the seventh plant only want interstellar credits?
Those odd creatures from Altair's seventh planet cared nothing for the welfare of the completely alien human beings; all they wanted was to feather their own pockets with interstellar credits, so that they could return to Vinau and buy many slaves. No Earth court could give an effective prison sentence to a creature whose life spanned approximately two thousand terrestrial years. Fortunately, because trade was more profitable than war, there had always been peace between Vinau and Terra, and, for that reason, Terra could not bar the entrance of apparently respectable citizens of a friendly planet.
 So that they could return to Vinau and buy slaves.
 So that they could buy slaves.  So that they could return to Vinau and buy slaves.  To buy booze any time they desired.  So that they could return to Vinau.