Eric, Bryan and other digID fans are asserting that markets require identity and reputation. This is bogus. One of the great things about markets is that you can trade with people you don't know or trust very well.
Try this out at your local farmers market. Walk up to the chap selling fruit & veg, and offer him cash for them. He doesn't know you from (AKM) Adam, yet he'll readily take your cash, and you'll buy his fruit (assuming it isn't mouldy or over-priced in your opinion).
The great trick here is cash. A medium of exchange. This is the original economic technology hack, and it is a remarkably good one, as it avoids the whole trust conundrum, or rather, offloads it onto a certifying authority. All the authority certifies is the money, not the parties to the exchange at all. Try paying at the farmers market with a credit card, and see how far that gets you (do take another form of ID with you too).
The succesful online payment company, PayPal, takes this approach, certifying the existence of the money, not the worthiness of the individuals.
David Chaum's work on anonymous digital cash should not be forgotten here. His key patents expire in 2005...