All I can write about this is caveat emptor. Even if you can verify that a fleet has the ability to "sell" you the ship, there is absolutely no way to verify that they will "sell" you the ship.
I offer you and any other prospective buyers the opportunity to meet me somewhere in game and see my T5 Fleet Ship, and the fact I am not in a T5 Fleet.
As I already stated, I too was suspicious beforehand. Two things swayed my mind:
1) I was allowed to see the Fleet Status before paying anything, and check that there were enough provisions.
2) During this period I took a look at the leaderboard and saw that the seller had a very high contribution to that fleet, numbering several millions of fleet credits. This told me that they were not some scammer but a person who is devoted to their fleet, and probably gave a good amount of time and resources into getting it to T5.