Elbert your right buying stuff in bulk can work for people if they can buy at the the right price =) i think it also pays off to specialize in a few things for example i buy a lot of old retro games and motorbike gear from carboots/yardsales i know my makes and brands and roughly know what i can sell something for before i buy it.
Also its always worth looking through ebay at items which say collection only, watch these auction as they generally go for a lot less and in the last few hours its worth messaging them asking can they post. you will find due to the auction ending and the bids not being as high as they would hope they will agree to post and you can make a last minuet bid and get for a bargain, wait for then to arrive re-list with postage and cash in on there initial laziness =)