If you as a buyer don't know much about programming and you buy one of these themes, are you dependent on the seller giving updates every time wordpress requires an update? When the platform requires and update, sometimes the theme no longer works properly… What if the Seller no longer is around? Does MOJO THEMES provide tech support, are they capable of updating someone else' work?
Generally, yes a seller will update their theme because they have the opportunity to sell. However, if you find that your theme is no longer supported you can always approach Mojo Support and ask if they can help. Though I would not expect to get free services, after all these themes are very, very moderately priced.
Your other option is to get a new similar looking theme, or hire a freelancer to upgrade your theme. Both of which can be done for very nominal costs.