eBay sellers that don't deliver

Have any of you purchased something on eBay that was never delivered by the seller?

Why do you think this happens?

I've always got what I ordered. Before placing an order I make a habit of checking the seller's recent feedback for any neutral or negative issues. If the seller has 2000 positive feedbacks in the past months and 4 negatives I tend to ignore the negatives. If however there are 10 positives and 5 negatives then I don't order from them.