Oh, this forum is really too crowded. My topic has slipped pretty. Please, you answer it...
The phrase passing trade is a euphemism for clients/customers that pass by on the street and trade money for sex. Synonyms: john (US), punter (UK).
EX: [Prostitutes] must all go and stand on street corners and pick up passing trade ...
EX: But the roundabout girls are still ... attracting only passing trade.
The sayings that is to say and that says something are not related. The latter saying is short for that connection of ideas says something significant:
It was a very messy business. So messy, in fact, it made Bouchard nervous. And the fact that Bouchard, a guy who doesn't scare easily, was nervous about the events that had taken place tells us how significant the danger or degree of danger involved.