Is it possible for someone to join a Fleet and put in all the work to earn certain gear...and then leave the Fleet and still have the gear?

Yes. If the settings are set to allow anyone to buy from the stores, someone could just join and raid the stores and leave again...

Not that raiding the stores is very economically viable... it's costing you a boatload of stuff for character-bound equipment.

Some fleets do allow for it in exchange for donations to their projects/bank

In my experience, most reputable Fleet's will require members to input at least 200,000 Fleet Marks before access to the Fleet store is permitted, to prevent situations just like the one you describe.

I assume you mean Fleet Credits since I don't know of anyone that has obtained 200,000 Fleet Marks.

We get about one person a week that joins our fleet, sees the permissions are set to not let them steal from the store and the bank because the guy who set it up isn't an idiot, then promptly leaves.

Always good for a laugh.

I was using the ds9 exchange and i watched someone constantly spamming for fleet invites. Over and over again. I turn around and realize they are standing at the bank console. I watched several fleet names pop up over their title. I just laughed to myself.

Exactly the same here. Tho we don't really have that BIG influx of new members .. but the fun is there, definitely!

Right, Credits....can't even tell this **** apart anymore :/