Anything that involves using CCP’s game (EVE online) to exchange RL dollars. Is 99% going to get sued/shutdown if you advertise this in public.
All illegal. Anything with RL $$$.
Like others said doing a giveaway (ISK, Plex, Loot) to the winner is the best way to go around hosting an event like that. Raffles are perfectly ok also. Do anything you want long as it doesn’t involve RL money/assets.