General rule of thumb for an item which is used (even if you think it's new, it's not unless it direct from a retailer as such with relevant warranty etc) is 2/3 or cheapest retail price. so £99 gun, would be worth £66. Some brands hold much higher value such as TM where you could get 90% still. But on clones/cheaper brands, it's much less. This isn't a negative dig, but a brief insight into 2nd hand airsoft sales.