It's not worth the drive if you're just going for fresh veggies. There's one or two vendors who sell produce which you could probably buy at Publix for the same price. One meat person who, as shah mentioned, is expensive and doesn't actually have the meat on premises (you have to place an order with them).
Lots of junk food, a little French stand that makes tasty crepes to order, and a few odds and ends. Lots of hippy stuff like soaps and candles and aromatherapy.
Isn't there a big warehouse in Tampa who sells produce both wholesale and to the general public?
EDIT: Josh, it looks like St. Pete is prepping for the upcoming Grand Prix race through downtown, so I'm not sure how that will affect traffic around the area.