I recently got a powersave device and I like to trade random people legendaries or shinies. My legit shinies that I cloned trade fine, and my powersaved Diancie and Lugia trade fine, but for some reason the Celebi won't trade. I've read that a hacked pokeball, shiny-locked, unreleased hidden abilities or pokemon ( like Hoopa ) are untradable, but I can't tell if that's the problem with this Celebi. I also have a Mew that won't trade through passerby trade.
Hacked Celebi can be traded as any other Pokemon as long as they "seem" legit. You have to remember that both Celebi and Mew are event-only Legendary Pokemon, so they can't have strange Pokeballs or movesets. And of course Celebi can't be shiny, because it hasn't got an official shiny form yet.
If you add more information (maybe a screenshot) of those Pokemon we can help you to understand what's wrong with them.
PS: It's worth noting that you usually can't trade Pokemon with HM moves with Passersby if they are in your team, so check their moveset in this case.