First, let me say I think you have a good idea. I think starting out, you should focus on niche. Considering your idea, the riche market you're going after seems to be quite broad (think of all the bakeries/cafes etc. you can find in just one major city).

By going niche, you can quickly become the "go-to" expert in the field. This will help with word of mouth advertising, referrals and especially Google.

Once you've mastered that model, you can consider branching out (including more categories) or duplicating the model for another niche.

If you go broad, you'll be competing with Craigslist, Amazon, Facebook, and a variety of other "generic competitors.

Great idea - good luck with it!