I don't see market going up for them any more. Users with Sky3DS carts can easily use the ROM. I would personally just keep it, but if you have no need then I'd recommend posting it in the ShopTemp area of the forum for another temper.
I'd keep it if I were you. Who knows what could happen to 3DS hacking? Maybe tomorrow Gateway will release an update that enable the emuNAND to support 9.6. When that happens I believe the need for Cubic Ninja will increase and by that time if you want to sell it, sell it