Thank you all for your replies, I learned some new stuff that I didn't know XD
Well, the problem is with the item "Yellow Jelly Rabbit Backpack" which it says: "Already Bound - 6 Backpack" even though I don't use it as I have replace it with a better one. I tried to sale it to NPCs and to Auction (as it doesn't say anything about "Can't Trade" this one) but without any luck.
I also have another "Yellow Jelly Rabbit Backpack" which this one says: "Can't be Traded - (Event Only)" which of course I understand that I can't sale this one
So it seems that I just have to "destroy" the "Bound" item then as I can't sale it
Again thank you all for your help and Stundor nice guide you have made