When you dismantle weapons, they get put back into your offensive / defensive page. Ie, whatever weapons and ammo you were using, are simply put back into stock.
To sell them, click TRADE - WORLD MARKET TRADING - PRODUCTS IN STOCK: BUY AND SELL.
Scroll down to the ones you want to sell, and continue from there.
If your weapons are deactivated they will only take about 25% of the workers needed to staff them when they are active. The remaining workers will become unemployed and if you have enough of them they can be used to staff more corps, making you more money.
The weapons also use less training ammunition and so cost less. But you can only reactive 10% of the total weapons in stock (not including those in garrisons or units) per month, and they will then need a full complement of workers to staff them.