Would taking a picture of an individual at a sporting event and then selling it as a poster be legal? Also what would I be allowed to put on it? Would putting the name of the business that sells the poster be legal or would that be considered advertising and illegal?
I dont know, but you got the papparazzi taking pictures of celebrities every day and selling them to rags, so I imagine that is legal, otherwise they wouldn't exist. But they fall under the "news reporting" Fair Use section for TM and copyright law. You taking pictures to sell as posters, doesn't.
There are photography contests that suggest asking individuals for permission to avoid further problems, but not necessarily explain them your plans. Slipxaway observation is another point of consideration, not only celebrities, sometimes a TV camera man may take a shot when you are walking down the street, picture which is printed in media or aired on TV with no legal implications.
There is a huge difference between taking a picture of a celebrity for "news" purpose vs. using an image for commercial purposes, such as product or company endorsement. Putting a company name on a poster without a signed release is something you likely can't do. Many celebrities (i.e David Beckham) also have trademarks in various classifications, including posters, calendars, etc., so any use without their consent would clearly be trademark infringement.