It's a scam only if the leader didn't deliver what was promised in exchange for the donations. Leader said donate and I'll give you rank promotions or gil. Members donated and got their rank promotions or gil. They got what they were told they would get. If they were concerned about the value, they should have gotten more specific details before donating.

The sale of the FC is a different matter. SE's been pretty vague on the rights of FC members when it comes to FC ownership compared to the rights of the FC leader. The most you can do when a FC leader sells the FC out from under the members is to report it and hope that SE will decide a scam occurred then restore the FC to the members.

Any time you have a FC leader or officers doing shady things like asking for valuable donations to the FC in exchange for things of minimal or no value, run. Those FCs never end well.