How much of it do you have, what condition, and how much in a hurry are you to sell?
- Pretty close to a set of everything, NWT, and not in a hurry, price it at retail, but you're basically waiting for a newbie who missed when IW had the release up, now want it desperately and only you have the set.
- Just the OP, or an incomplete set, or not new, -- and willing to wait for a while if there's no buyer -- maybe 1/3 under retail, and be very willing to bargain. Again, people missed it when IW had it up and now it's a rare collab dress, but the fact that they still have to save $$$ for another piece of the set, or the socks have had your feet in them before theirs, will likely make them unwilling to part with quite as much cash.
- Just the dress, not great condition, or plain unwilling to wait -- price it cheap, someone will probably take it off your hands and scalp it in your stead.