What blows me away about this game is how the fight engine is impacted by so many variables. Hiddens, sliders, skills.....it's pretty awesome to think about. The whole concept of hiddens is maybe my favorite thing. What a genius idea.
Maybe Mike putting the game up for sale will do more help for the MMAT community than ever intended. I can't speak for everyone, but I know I've sat back over the past couple of days and really appreciated the game and the joy (and frustrations - lol) it's brought to my life for the last 15-16 years. In a world filled with stress and misery, it adds an escape. A few minutes here and there where I don't think about work, kids, divorce, etc.
Yeah so it is pretty much a straight up IP sale. The things to do really are create a buying guide - similar way to a house report to let the prospective buyers know what they are getting - how the fight engine works, the user database et al. Your fight engine should be protected via copyright which is the bulk of what you will sell. You do what is known as an 'assignment' - which basically details out what you are selling. I am going to guess that you will be outright selling it and waiving all your rights to it.
Selling the domain is fairly straight forward to transfer it over to another account. There might be a fee or two involved in it but they are tiny.
Selling people's data - this is where you could run into problems. I have read the T&C's and the privacy policy and I don't think you are covered to do so. You might want to check with your own solicitor regarding that. You might have to issue an opt out email to everyone to have their details removed from the database giving plenty of time for them to do so before the transfer goes through. The repecussions of not doing it - yeah not good at all. This is probably what will hold up the sale the longest.
Those are pretty much the big ones you want to look into. Good luck with the sale and hopefully it all goes smoothly.