Basically for my shop all you have to do is offer me another event pokemon (that I don't have) and i'll be okay with the trade. Event pokemon are sometimes different such as they can have moves that you can't normally get or like come shiny (there's a lot more to event pokemon btw just not gonna name everything cause that would be a bit much). And from what i know you can't catch a mew or victini in the wild cause and event is required to activate them. You can get mew in the wild through an event but only in 3rd gen japanese emerald i think it was. For victini you either need a liberty ticket which isn't distributed anymore or have gotten it from the wifi events a few months ago.