Do NOT trust the GTS for such things. Go to the trading board.
Ask for it, and see the sort of thing people want. The usual things people want for stones are: Other stones Shiny pokemon Xerneas/Yveltal/Zygarde with good nature/ivs Master Balls Pokemon with 4ivs or more
I actually have one I don't want though so if you want to make an offer...
I really have nothing to give besides extra charmanders, squirtles, chespins that I hatched from daycare.
As I said, get your style up and go to the Evolution Stone Boutique in Lumiose. A stone will only cost you 10K if you have max style, and you can easily get style up by buying Premier balls, one at a time (get like fifty and see how you're going before you continue buying more).