Because it would force people to be honest in order to eat. Economic activity as a whole would become a lot more transparent because people will avoid hiring you or buying from you if you have a bad reputation. The inverse is also true, rewarding the business owner who invests in quality and customer service.
> if the merchant makes a run for it
Again, this is a discernment issue not a systems issue. In that particular case, you can set up an escrow transaction that only releases funds if the transaction goes through. EBay has already proven, too, that most people are honest by default so this is a non-issue.
> In a world of non-reversible payments, I'd probably stick to Amazon exclusively. That seems pretty bad for small/new/independent merchants.
That's a personal choice.
