Is this legal for a website?

I am building a website that gives people the chance to buy a 50X50 pixel square to advertise their site. Each square is $100. I want to hold a lotto or raffle every time $100,000 is collected, the prize will be $10,000 and only people that have bought space are eligible for the prize. I want to know how I can do this 100% legal. What kind of licenses do I need. The prize of $10,000 will be taken out of the money collected so the lotto/raffle date is at random until the $100,000 is collected. Is this legal.