I think for a smaller shop / startup the easiest way to go would be to have users enter their own google adsense client number and use that a certain portion of the time.<p>By making them sign up for Google you get G to do the footwork of verifying they are eligible, and managing payments...etc
Take a look at what Revver has done with user accounts. Or Jellyfish.<p>There's usually a float involved which can be fun for cash flow (time between affiliate payout >= time your affiliates need to be paid).<p>Some companies won't let you / have rules about compensating users. For ex: WalMart doesn't like it, on the grounds that they feel their brand is strong enough to attract people without affiliates having to offer a monetary incentive.
Well, just give it to them.<p>Cut checks, use PayPal, or Amazon FPS.<p>Accounting would be your biggest hassle. You need to file a separate 1099 with IRS for each person you pay more than $650.