what you install will never look like what you see in the demo unless you have the same content and options. that is one of the realities of wordpress themes
thought I posted this yesterday, Free File Of the Month is a wordpress theme, take a look at it
in fact, the July 2012 Free File Of the Month is a WordPress theme
check it out, download it and study it
to answer your question, there is no definitive list of what is needed to write a theme for sale on Themeforest. one thing for sure is that that minimalist theme would never be accepted. good luck with your ventures and if you can ever nail down the requirements for a theme for sale on Themeforest, everyone will beating a path to your door
p.s. also forgot, every once in a while, the Free File of the Month is a WordPress theme, so keep your eyes open for one
the annual bundle sale is coming up
usually a WordPress theme is included, buy this and get good value along with a popular WordPress theme and check out how it is created and documented
may be your best value for your money
if it is a public slider, google for it
if it is for sale slider, you would need to buy it or use another slider, not so sure the actual slider being used is that critical, you can probably adapt a public slider to do the job
what software was used in the gallery app? is it a priority code or is it public code?
if it is public code, then you can find this code and implement it yourself. for example, if the nivo slider is being used, that has not been created by the author of the theme/template but free versions of it are readily available which for example he may have used in his product. after all, if the code for the app is publicly available, he is using it for free, and he doesn’t pay for it, why should you?
check out my link
Jeffery Way did an article to create an HTML template which he did promise to convert to WordPress. here is an elementary example of his template converted to a WP theme
perhaps I need to expand the articlle to include the steps to set it up as a WP theme, it is not complete but does do some of the things that the HTML template does.
let me know what you think
ps Chric at css-tricks.com does have some videos where he has converted HTML templates to WordPress themes, check his videos out and they are free