Massive difference. I have to actually hand it over in person.
If I walked into a store and purchased a book with cash, and the cashier didn't give me the book I would cause a huge uproar.
If I purchased a book on an online store and paid with Bitcoins and it didn't arrive in post, who do I turn to now?
Problem with reputation is a new store begins at 0, so the first customer has to take a leap of faith.
OK - So what you're talking about is online transactions but people can and do use bitcoins to pay for things face to face. Bitcoins are also being used to purchase digital goods and services with reputable retailers who provide immediate delivery. This forum is sponsored by one, Private Internet Access.