 Why did the odd beings from the seventh plant only want interstellar credits?
As for the extraterrestrials—it was a free bar—they were merely amused, since to them all men were pathetically and irredeemably hideous. "You must allow me to pay your cleaning bill," Gabe said, taking out his wallet and extracting several credit notes without seeming to look at them. There he inserted a coin in a locker, into which he put most of his personal possessions, reserving only a sum of money. Each time it happens, there's a ... well, there's no experience to match it at a thrill-mill." Only when she started to struggle out by herself did he seem to remember her existence.
 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.