eventually when you get many subscribers and visitors each day and or you exceed the mb disk space of the server that hosts your subdomain its time to transfer to a host

you can save some money and transfer a sub domain to a subdomain

or you can spend money to make money by transferring your domain into a dot com or whatever that you should make shure the company lets you register and keep full rights of all aspects of your internet realestate along with supplying you with 24 7 up time and long band widths load times and services that make web building easier along with buying and selling services

buying and selling may make sense if you are a web firm that hires people to do the dirty work but as an indivdual the emphasis should be on quality foremost and the maximization of your one site sale or you will be wasting alot of time and energy just o get ostracized online for being a spammer