The sell value of an item is half the store price of any weapons and relics. Sell values of weapons and relics bought using karma are also half the buying price, rounded down. Gift and Tournament weapons' sell values are very low, as they required no payment to obtain.
For example, the Laser Visor has a sell value of 5,000,000 Gold. If the laser visor was sold in stores, its price would likely be 10,000,000 Gold.
Note: The laser visor above displays the sell value prior to an update decreasing the value.