I've personally made a lot of modules myself for mybb 1.2's myplaza, so I finally decided to release one =)
1) Open theme.php and change "___" to the name of your theme
2) Change the "4" in "db_update('users', array('style' => 4), 'uid='.intval($mybb->user['uid']));" to the theme id you wish to sell.
3) For any additional themes, copy theme.php and rename to something like "theme2.php", then in the file change "function theme_" to "function theme2_" for all 4 functions.
Pretty basic, but the users at my forum love it
Also don't forgot to make the theme you want to sell hidden from all usergroups, or else there isn't any point in them buying it.
Demo:
http://brawlfriendcodes.com/plaza.php
(under Forum section in shop)
Enjoy