So I have a mew in Alpha Sapphire, I believe it is completely legit, but I can't remember exactly. Some people on the GTS want a mew in exchange for a shiny Arceus. Sometimes, he'll (it?) be holding a plate. What I want to know is if that would be a good trade or not.
I'd say it would be fair (well, maybe not for the receiver) if your Mew was normal but the Arceus was shiny. But really, it's up to you.
In my opinion, it's a bad trade. Shiny Arceus are all over the GTS, I've owned about 5 of them at one point or another. Mew is MUCH more exclusive; there are zero on the GTS and they are very hard to come by.