I set out to sell my nyx because I'm not using it and could use the isk. Originally I was thinking darkness or chribba, but they weren't available while me and the buyer were on and we agreed to use somer's third party service.
The buyer put up a passworded pos in the system. He and somer warped to the pos; he transferred the isk to somer; I warped to the pos, ejected, and he got in. He verified the fittings and somer transferred the isk to me.
It went down like this (edited for brevity):
Buyer> isk sent
Somerset Mahm > Confirming I've received 23b ISK. You're clear to join fleet
Autumnries Mahm > what's the POS password, please?
Buyer> clr to warp 0m to me
Autumnries Mahm > POS entered successfully
Me > warping now
Me > Okay. If it's a trap and the nyx dies, I get the money. If it's stolen by a mysterious third party who has the password, I get the money. If the fit is wrong, I get the nyx back.
Autumnries Mahm > correct :P
Autumnries Mahm > on all counts
Buyer> i am ready to enter
Autumnries Mahm > Okay, I see Nyx in shields. Buyer: again, please verify that everything is correct, as this is your only chance to do so while being secured
Autumnries Mahm > Since this is your POS and you won't leave immediately, once you confirm everything is okay, I will release the money
Me > Oke, ejecting. Don't press self destruct, don't press self destruct....
Autumnries Mahm > okay, Nyx is empty
Autumnries Mahm > and now Buyer is in it
Autumnries Mahm > Please check fit :)
Buyer> in the ship
Buyer> everything is here
Buyer> thank you very much
Me > Thank you.
Autumnries Mahm > okay, great!
Autumnries Mahm > I'm now sending the money to [me]
Me > I have the isk