Easy -- the FAA Chief Counsel has issued interpretations saying that the flight time itself is compensation, and even that in some cases (when there is a prior/existing business relationship between the parties involved), the goodwill created constitutes compensation even if the pilot pays all the costs.
Exactly. But that's not quite the same as saying "no money is changing hands for flying."
...as long as there is no quid pro quo involving the pilot services. The money paid must be for that alternate service, and if it appears excessive for that service (say, $200 for sweeping out someone's T-hangar), and the FAA finds out, the sweeper/pilot is toast. More than that, the attempt to conceal the transaction would be treated as evidence of willful intent, and the penalties will be more severe.
All this said, unless the FAA finds out, they won't care. How would they find out? Let's say you provide such a ferry service, and prang the plane. The FAA Inspector doing the investigation (the NTSB almost always delegates light GA accidents to the local FSDO to investigate) goes back up the trail checking fuelings (they do that on a lot of accident investigations) and find out the owner, not you, paid for the gas at each fuel stop. Or, you delay en route for weather, and the owner, being very annoyed that you've not delivered the plane as agreed, complains to the FAA that he didn't get his money's worth from you. BTW, that's one of the two big ways faux charter cases happen -- the passengers complaining to the FAA about not getting what they paid for. Either way, it's "Adios, amigo."
So, "You've got to ask yourself one question: Do I feel lucky? Well, do ya, punk?"*
*"Dirty Harry," HJ & RM Fink, 1971.