It may be pointless to begin with. Most supermarket produce is from hybrid plants and the plant you grow from those seeds will give the same tomatoes, the plant will revert to some earlier form. I've had tomatoes fro the supermarket grow in my compost, they made strong plants but with a few tiny tomatoes that were worthless. Tomatoes labeled as heritage or organic or from a Farmer's market may give better results.
Raising peppers from supermarket peppers may give you a decent pepper, there has been less hybridization done to peppers BUT
Green bell peppers are not ripe, so those seeds may not grow at all. All green peppers turn to another color when they ripen so you might have some luck with seeds from red bell peppers (or yellow etc)