OptimizePress

Well, selling a website means that you give full control of the website for the new owner. You can choose if you would give them the full hosting account it is currently on (provided its the only website in there) or you would transfer the whole content to their host. Whichever the scenario you are transferring everything - files and databases, which includes Optimize Press. So, as long as their host is compatible with Optimize Press (which they should) then it doesn't matter if the new owner have installed it separately or not.