Where would you sell a antique bb gun?

First I would find out the value by asking here on "Answers."

State the Make and Model. Then give a brief description of the condition of the gun. State if it is working or not (Does it hold air or C02.) Then give it some sort of ration like Poor, Fair, Good, Very Good or Excellent condition.

Then I would try "Gunsamarica" on the web or "Craigslist" on the web.