 Why is Retief so concerned about the tractor order?
"Breaking and entering," Retief said. "Our function is merely to bring them together. I'll leave him to you, Retief," Magnan said. Our furniture—" "I've seen some of your furniture," Retief said. Arapoulous puffed on his cigar, looked worriedly at Retief. "It sounds like I've been missing something," said Retief. Retief looked at the two squat bottles, one green, one amber, both dusty, with faded labels, and blackened corks secured by wire. Retief picked one up, sniffed the deep rust-colored fluid, tasted it, then took a healthy swallow. Retief put aside his cigar, pulled the wires loose, nudged the cork, caught it as it popped up.
 Because the order of tractors is unusually large.
 Because he knows whoever ordered the tractors has bad intentions.  Because he knows the order is a mistake.  Because the order of tractors is unusually large.  Because no one else appears to be concerned about the tractors.