I have an old Encore pistol with a 12" barrel topped with a 2x7 scope. If I was going to sell it I have no idea of its worth. Any suggestions?

Depends entirely on the quality of scope.

The used barrel alone is probably $200-$250.

Didn't mention the gun is stainless. The scope is a 2.5 x 7 Thompson Center pistol scope. Rubber grip and forend.

Ill give a guesstimate/opinion.....of $550-$600. May depend on area, time of year, or even audience advertised to. Of course the cheaper the price, the faster it will move.
Tax return time, or a bit before hunting seasons may be preferable. Parting out may bring the best money.

IF YOU PRICE IT AT WHAT IT MAY BE WORTH YOU WILL NEVER SELL IT. YOU NEED TO DECIDE IF YOU WANT TO ADVERTISE THE GUN OR SELL IT. IF YOU WANT TO MOVE IT YOU HAVE TO PRICE IT RUGHT.