My wife used to do this. Mariposa coop may be a good place to look (but they may only sell goods from members) and Sam's also used to be a good place to sell homemade goodies.

In order to sell goods in Philly, legally you will need an inspection of your kitchen by the health dept, a business license for the city (I forget what its called exactly) and possibly a certificate that you completed a training course about food safety.

Most Farmer's Markets will require proof that you've satisfied these requirements before you can sell.

These guidelines also only generally apply for folks who ask for permission