Do you have a market built? just having a tailor, gunsmith, gen store allows you to buy sell items back to those stores. But to auction items off that has to have a market built in town.
Once that is done you have to be in town to sell the item, and you have to put prices on the items above their sale price and I believe you must have enough cash to cover the sales fee when you list it (i might be wrong on that last one, it might be collected out of the paid price)