I traded with someone to give them Items (Heart Scales) but they stole my Pokemon.and the Pokemon were really valuable. is there a way to get back the Savedata before I traded?
If you traded, then that means the game saved to prevent cheaters from doubling their pokemon. I'm sorry to say this, but there's nothing you can do except trade for the pokemon back again, or recapture them.
No, I have done the same thing and so far ive never been able to get the saved data back. I wish I could help more but the next choice is either find a cheat forget about it or go back to that person and if he trades do the same thing back to him.(Thats what I would do.)