From one BA owner to another - Black Advents are very rare and they are at the top tier of the
September List. It's very high in demand: that's why you're getting all of these insane trades for it. If you look at the hierarchy in the thread I linked you to, you can see that it is quite high up - the entirety of the Advent List is below it and it's almost on the Main List (where all of the really big rarities are and all the pets that are the most rare and the most wanted are located), though you would have to do some swapping and trading up if you'd want to go further. Whether you're happy as you are or you'd like to trade futher with it, I'd definitely hang onto it if I were you! I'd check trades very carefully so you don't accidentally accept a trade that may have been unfair to you - use the list if you get stuck.
I'm not sure what you mean by the one with the "shima pattern", sorry! Hope this helps anyway. :3