Any potential problems look out for? What are buyers generally looking for in a theme? What should you include with your theme (documentation and such)? Any other advice would also be greatly appreciated.
If we are talking about HTML or CMS based Theme Development then these are my thoughts...
I tend to look for a well documented theme, one that comes with the Photoshop or Illustrator file as default, and if I am lucky, where the Logo (header and footer, if required) are separate Photoshop or Illustrator files.
My reason for that is, because I have purchased a few themes from ThemeForest, and it annoys me when the Logo is in the Main .psd file along with the rest of the design.
Some people just want simplicity, and that is something to bear in mind.