I have a differing opinion from most, I say go for it!!!
Partly because I want to hear the conclusion to this story, who knows, if it turns out to be the sister-in-law, (I assume it's not) maybe she'll be down. Plus, imho, ruining a marriage is nothing in the big scheme of things

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My only story about knowing an SP:
A few years ago I was living in a rough area in North Mississauga, on the Brampton border. One day I realized my
next-door neighbor had ads on LL.
It was weird to see it from that perspective because I saw how rough she was looking and sounding in regular everyday life and how misleading the ads were.
It migh'tve been interesting to share that with my GF at the time, as she already knew something was up with neighbors. But I chose not to share it, mostly because it would be hard to explain how I spotted her ad.
I don't want to cast too many judgments and mostly just feel sorry for her. But she seemed like a girl that was into some hard drugs and headed downhill. They were not good neighbors. It was her, a sister, and the sister's boyfriend. I remember one late night, during the regular blowouts at 2am, the guy yelling "can you stop masturbating on the lawn!?"
Also once saw her dealing with some problems from cops due to a car call in the parking lot.