Amazon Ebay sellers

I am a bit confused. I hear about selling on Amazon and Ebay and how the big time sellers are making big money. Then I hear how these people need financing to maintain inventories or for other associated needs.

If you are making big money why do you need financing? Is it because it isn't as lucrative as advertised?

Obviously there is a disconnect. For me it is one dimensional but for a guy w/a larger IQ it is a piece of cake. If anyone can explain it, I'd appreciate it.