Hey All! A question that needs to be dealt with before I go berserk

... Say for example I'm selling mobiles, I start off on eBay but there's only so many people who want a mobile right? No one except for other sellers are going to buy more then 1 of a mobile.
Where can I branch out and take my business, say I'm working on selling 30 a week, is there somewhere I can branch out to sell more or should I perhaps continue doing like a few mobiles, and do some different items too?
Also is there a way to check like the most sold items on eBay, or the sections of which most item sales happen in?
Thanks in advance guys!