When you try to sell something on the market without having the money for the 10% fee, then there is an error message something like: Putting these items on the market would cost ###c but you don't have that much.
I wanted to buy something off the market. I pressed buy, next page started to load but it was loading too slowly. So clicked at buy a few more times. But system remembered all my clicks and I bought a lot of that stuff even though I din't have enough money for that.....So my cash stat started with - then.....
I remembe reading somewhere that if you have no cash and move items to a store for sale, you'll be "loaned" the transport costs. I've never done it, but it makes sense that it would give you a negative balance.