To answer your question on why people need bitcoin so badly, is the same reason people will pay %20 - %40 at times on LBC and other websites. There are many gamblers who lose and in frustration need to obtain Bitcoin as soon as possible to fill their problem. People also may need to pay a loan off and can't wait a week for Bitcoin to arrive from their exchange. Other people may not like the invasive information many exchanges require, which results in those users paying high markups to obtain their Bitcoin.

In regards to the main topic, a seller of Bitcoin on ebay will lose 12.5% of their total sale due to fees. Right off the bat, a seller is going to automatically markup bitcoin 12.5%, and then on top of that, add anywhere from 30-40%. If someone does 10 trades, and 3 people attempt chargebacks, that extra 30-40% per trade would still allow the seller to atleast break even or make a little bit of profit at the end of it.