if you want to get the json just use one of the following links:
general steam inventory: ( 1: games, 3: coupons, 6: trading cards, 7: rewards )
http://steamcommunity.com/id/<USERURL>/inventory/json/753/1
http://steamcommunity.com/profiles/<STEAMID>/inventory/json/753/1
http://steamcommunity.com/id/<USERURL>/inventory/json/753/3
http://steamcommunity.com/profiles/<STEAMID>/inventory/json/753/3
http://steamcommunity.com/id/<USERURL>/inventory/json/753/6
http://steamcommunity.com/profiles/<STEAMID>/inventory/json/753/6
http://steamcommunity.com/id/<USERURL>/inventory/json/753/7
http://steamcommunity.com/profiles/<STEAMID>/inventory/json/753/7
games :
http://steamcommunity.com/id/<USERURL>/inventory/json/<APPID>/2
http://steamcommunity.com/profiles/<STEAMID>/inventory/json/<APPID>/2
the problem was not the counter-strike appid,
you just used an old link that only works for the general steam inventory.
I suggest you using the link using STEAMID because not every steamuser has set a customurl.