There were about 1500 original Kialaras made. With a low serial number you should be able to get about $1200 for it. Not having the original sleeve does bother some collectors though. Asking 0.21 btc is a bit much, for that a buyer could get a Silver Kialara whose mintage was only 250. The originals are not as prized as the Silvers or Golds. Just the way the market works. Also the market is a bit soft right now. I agree with others that you should start an auction using one of the trusted members here like minerjones for escrow. Good luck with the sale!
Yes saying price in bitcoin doesn't work so well it changes fast. You say $1200, OK that's food. I will try to maximize some. I say who gives me $1300 in bitcoin or Swedish currency can pick it up or be sent it after payment. We can do deal over video-app. I show coin/artpiece and QR to camera, state verbally what is to happend and you send. This way we have clear recording with blockchain evidence for legal claims. I am Ayn Rand level honest though so will not be problem.
You say escrow. Does one send the piece to escrow for reshipping?
With escrow, you both agree on using a reputable member on here (there are a few to choose from).
The escrow holder provides the buyer with an address to fund.
Once payment has been confirmed, you send the item straight to the buyer. Once they receive it, they ask the escrow to release the funds to you (the seller).
If there are any issues then the escrow evaluates the dispute.
For an item of this value it is highly advisable to send tracked (insured would be a good idea too).