Legality of a flight

All true, but there is no specific "Angel Flight" exemption. Our
flights are purely part91 flights with no "compensation" going to
either the pilot or the organization in return for the flight (neither
the patient nor the hospital is ever billed a dime). The FAA has said
that they would not count any "charitable tax deduction" claimed by the
pilot as "compensation" - but that is a "commercial" issue and not
relevant to the legality of the flight itself.

There is work still going on to allow groups or individuals to
"sponsor" a flight. This would, for example, cover a bunch of company
employees that want to put something towards a co-worker getting to
treatment. They can do that now, but it's not specific to that
individual flight. That (and even letting them help pay for the gas
for a flight) are being worked, but not definitized yet. [Sort of a
"share the costs" which is already allowed, but the others sharing are
not the passenger.]

The original question here has come up before, and not with a perfect
answer (i.e. you get as many answers as FSDO's you call and ask).
Clearly a charity fund-raising flight day has specific rules (advanced
notification, testing, etc.). This is the "Come to the airport on
Saturday... airplane rides for sale all day... all money going to XXX
charity." type of thing.

But this case (original poster) is more the "I'll let you auction off a
ride in my airplane as part of your fund raising activities, and I'll
give whoever wins a ride sometime later." Frankly, usually it's just
"done" and no one really cares if so and so gets an hour of sight
seeing from the air. I do suspect that it would be looked at much
differently if you were to offer "a trip from LA to SF to the highest
bidder." There you are clearly providing air transport rather than
just a sight seeing trip.