I am not a lawyer, but I play one on the internet......
If you are selling photos of people at an event to the people who were in the event , you would not need a model release. If you use the photos for your personal advertising you MAY need a model release. Most of us do not get one. If you use photos of people to advertise Coke or vacuum cleaners you NEED a model release. We shoot events all the time, give the sponsors photos of the event, and sell the individual photos to those interested.
Take a look at Exposure Manager, they are a TPF sponsor. Their interface is not horribly complicated, and they give you the option of printing the photos yourself, or having a commercial lab print and mail them for you. You set your own prices, packages, etc.
For example
click here and go to ballet and pw is margo
They are very responsive to inquiries, and I have never had a quality complaint .