If you wanna buy cheap nx online, (not from nexon website) then go along to this website OffGamers Online Game Store - MMO Game, PC Game CD Key and Game Card Store. and buy their "Ultimate Game Cards". A $100 USD card will cost you $100 USD unlike other sellers that charge $120-$140 for 100k nx.
Also for sale
$10 card - 10k nx
$15 card - 15k nx
$20 card - 20k nx
$30 card - 30k nx
$50 card - 50k nx
$100 card - 100k nx - (my bad typo, correct)
All prices cost that amount and no more or no less, its just like going down to the store and buying it, you can pay through paypal too and credit card and a few other ways.
To use "Ultimate Game Card Codes" you need to choose the "PayByCash" option on the "GetNX" page and you will probably figure the rest out.
After being verified and all that other stuff, after purchasing your items from this website within 5-10 minutes you will receive your game card code, 1st time takes alot longer under an hour usually.
"Many Thanks".