I mean, given that johnscardsforsale, a very rich neopian, used to do that and got very very rich off of it, I would imagine it to be legal. He might still do it, I really don't talk to him anymore but I know for a fact he has turned in hundreds of codes for the items.
A little more research shows that yes, it is legal. TnT has never commented on it as far as I know of, but Jellyneo does have a page outlining the use of RICs and I can't imagine why it would no longer be allowed. The idea itself seems shady on TnTs part, but honestly, they have proven time and time again that $$ > Rules.
On a final note, it may be worth your time (and money) to buy from users on cK if you are looking for neopoint purchases. While it may be against the rules, many users do in fact buy neopoints and get away with it. It is more cost efficient as well, and you are probably just as likely to be banned for a nonsensical reason as you are to be banned for buying neopoints. Just my opinion however, feel free to PM me with any further questions.