Or what if we do this? When they join the forum we can let them choose three ip's of their choice and we can make a rule to only use their account from those IP's. By doing so we can stop them from selling their accounts right?
You're assuming that everyone has static IPs, for a start. Many people don't and therefore have no control over what their current and future IPs will be.
You're also assuming that people log into the internet from only a few places, when the reality is that a lot of people are connecting from multiple different places on any given day, all of which have different IPs. It's not unusual for someone to connect to the net from home, work, their phone, various sources of free wifi, and the homes of friends or family all in one day, and even more different locations if their work takes them to different places or they're travelling.
Yeah, What if we tell them beforehand that they can use their account only from certain places. How would that affect the forum?