Nothing Protects You Like Pam's Song!

anything else i buy with money in the real world can be traded for real money at any time i choose; it retains value

So if I put my coins in the box to get on the bus, I can get them back?
If I pay for a ticket to a show, can I trade it back into money after the show has passed, whether I went or not?
If I buy a haircut, I can trade that haircut in for money?

There are any number of things in the real world which you pay for and cannot trade for real money afterwards. The closest to NX cash is gift certificates, although I'm not sure what the laws are regarding those in your jurisdiction. Where I live, most stores will not give you money for them.

There are also any number of things in the real world that you pay for and get nothing tangible in return: all manner of entry or participation fees, such as to the movies, concerts, ballgames, museums, guided tours, cruises, amusements parks, and so on and so forth.