[trounce]

Contrary to other comments, splitting costs is perfectly legal for private pilots under FAR §61.113(c)

The fact that an intermediary allows the pilot and the passengers to find each other, or acts as an escrow service to ensure payments are made, is irrelevant.

§61.113 Private pilot privileges and limitations: Pilot in command. (a) Except as provided in paragraphs (b) through (h) of this section, no person who holds a private pilot certificate may act as pilot in command of an aircraft that is carrying passengers or property for compensation or hire; nor may that person, for compensation or hire, act as pilot in command of an aircraft.

(c) A private pilot may not pay less than the pro rata share of the operating expenses of a flight with passengers, provided the expenses involve only fuel, oil, airport expenditures, or rental fees.