Selling photos at the race track

LRussoPhoto

I have started to take photos at local motocross races. I've been selling them on Smugmug and been doing fairly well. I see that there are some photog's at the track with a printer to sell the photos right there. I have had of photog's tell me that thats the way to make real money because people want there photos right away. Any suggestions on what type of printer I would need and how much they cost? Do you agree with this method of selling photos?