You can sell your Sta.sh deviations as a Premium Download or Exclusive directly to buyers by clicking "Sell Privately" on the unlisted deviation page. Once listed as a Premium Download or Exclusive, the right side of the Sta.sh deviation page will include the sale component for visitors to peruse and buy. 

Set Sta.sh items for sale privately from the Studio or on the Sta.sh page itself.

You can always submit or schedule a Sta.sh deviation publicly using "Edit or Submit." If it's been privately sold as an Exclusive, however, you should ensure you have the buyer's permission.