To sell an item, go to Garage - Items - tap the item and select Sell.

In the case of Wild Cards, you can only sell them directly for gold at the price offered by the game. For Blueprints (except Basic White Blueprints), Upgrade Cards and Redraw Cards, you can put them on the market for other players to buy. To do this, tap Sell as described above and select Market. You will then see a screen where you can set a price.

To see what items you have put for sale, visit Garage – Items and check "Sale" tab.