the big secret is be prepared to deal in new phones as they come out, dont buy too much stock of one type and be left with it if prices drop.
Also a HUGE tip!
DON't just look at selling on eBay. Look at other options: newspapers, trade sites, markets, smaller websites etc.. You may not get as much traffic but you will be able to get a much high price than eBay as eBay has the most competition in the world!
THINK OUTSIDE THE SQUARE! ;)