Several points:
1) St. Lawrence Market in Toronto is great ... I would stop there on the way to my family's cottage in the Rideau Lakes region. However, a more interesting market is the Kensington Market which is west of Spadina St. south of Bloor. BOTH are well worth a visit.
2) The West Side Market - the inside portion with the butchers, cheese, etc., is a great market. The produce which is sold outside is a very hit and miss proposition. Some stands sell quality; most sell surplus. You can do *WELL* there if you know who does a nice job and who sells garbage. (Disclaimer: I have a lot of friends who own stalls and stands at the WSM. I shopped there every week I was in Cleveland.)
For the record, if you relocated the WSM to Chicago, I would not buy my produce there as there are some great produce places like Eurofresh , joseph's Marketplace, and the like that offer better quality.
3) The BEST overall market is the Eastern Market in Detroit. They have surplus produce, dozens of real farmers from Michigan and Southern Ontario, and a lot of wholesale meat markets that sell to the public. There are vendors that would keep me stocked up with fresh spices year round there. They also have sell a lot of flowers, Christmas trees, etc.
Since you are dealing with farmers and surplus dealers, they are motivated not to take product back home. If you arrive at 3:00 pm, you can get some phenomenal deals - cases of zucchini, asparagus for $2-3 ... if you can or freeze, you can get some great steals on produce ... and the quality is generally pretty good.
If you can find the new location of the Windsor, ON public market (it was relocated by the casino), that is well worth a visit.