The market here is high. $175 isn't a terrible asking price. This bike has been on C-List on and off for weeks. I'd go and have a look and a ride.

Bring a tape measure. Looking at the big chainring in the pic, it should be about 6 - 7 inches in diameter, assuming a 46 tooth. Double that and add a couple inches, and I believe the seat tube is indeed 16". Visualize the top tube as being horizontal and measure the intersect point on the seat post, just above where the rear light wraps around. You'll likely find that the bike is really a 20" or even a 21". Perfect for a person who is 5' 8".

Offer $145. Settle at $155. Decent bike. Fabulous tires if they are Marathon Plus.