Selling photos to people - legality?

Is it legal to take pictures of people at an event, then hand them a card with a web address where they can go and buy it? Am I exposing myself to liability if I don't have them sign a model release first (which would be impractical)? What about if someone other than the people in the picture bought it? I wouldn't be able to tell if the person buying it would be the person in the picture. Would I need to keep them private?

Basically, I'm trying to decide if it's worth the risk for minimal profit.

We have people do this at our events all the time.

Consider this general advice from a non-lawyer:
Unless you're at an event where they don't allow photography, like a NFL game or concert etc., you can sell any image that you want to to anybody who wants it. You can even sell the rights to publish it.
But unless it's a news use (& there's more to it than that), in order to publish it for a commercial use, the publisher needs a release from the identifiable people in it.
When do I need a Release?

Actually, you can sell pictures even in those circumstances, if you manage to get away with some pictures in your camera.

In the US, the power of copyright combined with the First Amendment right of self-expression is extraordinarily durable for sales of individual photographs, blocked only partially by rights to personal privacy and personal publicity, but hardly at all by property rights.

There is almost no circumstance that photographer A is prohibited from selling a picture he has taken to customer B. Even if the circumstance in which he took the picture was illegal--such as trespassing--he can still legally sell that picture. He might go to jail for trespassing, but he can still sell the picture.

Very interesting information...

A book called Starting Your Career As A Freelance Photographer by Tad Crawford talks about situations like this. Once someone goes to an event that pictures are able to be taken or in public events the person gives up rights to any picture being taken of them. So you can do whatever you wish with them. So yes you can sell them to who ever wants to purchase them. Just make sure its ok with the people throwing the event if its private.