I've been selling on eBay professionally for 20 years, so I suppose survival alone makes me reasonably successful!
There are so many things that can trip up even an experienced seller, but borrowing money to start up is one of the most common. There is also pressure to borrow against future income (used to be from PayPal, now it's YouLend). Lots of people think it will be fine, and overestimate their profit margins, so when 25% (or whatever their rate is) of their takings disappears every day it can become difficult to buy in new stock, or pay other costs.