Couple of ways, both very different to each other.
Firstly, there is always a lot of sales just waiting to be made when dealing in the hot end of eBay, those hottest selling items. One of the methods used there is a large volume purchase (ensuring the lowest possible per item price), then working on a very thin margin and providing a good reliable service to ensure you remain super competitive.
Secondly, you can list a volume of items in an eBay store platform, which of course can even be done by using a drop shipping model. Being competitive and providing a solid service is still important, but a good sales volume can be created by providing a good stock profile.
Now bare in mind that I have outlined two volume selling techniques here, but that doesn't mean that they are both a fast or cost efficient way to make money, if in fact they make money at all!
Remember, just because someone is making sales, doesn't mean they are making money. You have to take in all factors when choosing any selling method, then make your choice as to what is the most effective for you.