The lure of buying NeoCash cards lies in the extra prizes awarded on redemption. Every card gives out a special item when you redeem it - initially these were NeoPoint wearables, though TNT later rethought this as now the prizes are NeoCash wearables - as well as Basic Gift Boxes which can be used to send NeoCash items to other accounts. Quite useful for trading with other users. The $25 NeoCash cards also award an extra 150 NC free.
once a day..other than that you have to buy through a credit card or buying a neocash gift card (target dept store exclusive) or possibly a random event
You ARE allowed to redeem Neocash cards on any of your accounts, although any item you receive as a reward for doing so should be sent to your main account to be sold. (By this I mean any non-NC Mall items that you are given as a reward for buying a Neocash card, such as .)