Do you do marketing to sell digital products on your sites and do you offer any type of advertising on them?
I presume you're asking people who sell such digital products. Not every webmaster sells digital products. Some sell real physical products, furniture, electronics, beauty, health products, clothes whatever etc etc. Would you still like to know if those people apply to your question also or are you wanting to ask people who sell digital products only?
If so what are the differences between internet marketing, selling and advertising?
Well Internet marketing and advertising are about the same sort of thing. Selling is what takes place after. You have to focus on doing all of them if you want to be a successful website.
and what factor do you want to focus on? what do you guys think?
I'd rather focus on just the selling but in order to do that, first I have to focus on the Internet marketing and advertising before any selling can even take place that's what I think.