Stem-end cracking of tomatoes – sometimes called Growth cracks – happens during the last phase of growth and ripening. It is usually caused by extreme conditions, such as hot weather, dry followed by wet, or lack of nutrition followed by a high dose of nutrients – especially nitrogen. Any of these can cause too rapid growth during ripening. Some varieties, such as the beefsteaks, are thought to be more susceptible to cracking than others.
Anything you can do to maintain even growing conditions will mitigate this condition – such as watering often, so that the ground never dries out, and small applications of a balanced fertilizer applied often. Some folks also put shade cloth over tomatoes during the hottest conditions. Fast growth during extreme heat may cause some cracking in spite of your best efforts, but always remember to maintain ideal conditions in every area within your control.
Cracked tomatoes are edible, and should be used before infection by decay organisms enters the fruit and rots it.
If you are trying to sell your tomatoes, and they continue to crack despite all you can do, you might try picking them just as they begin to change color, and before cracking becomes a problem. Finish the ripening process in a cool, dry place, then package them for sale. This may reduce the flavor somewhat.