You're correct. Not too long ago users were able to purchase name change tokens with credits from other users, regardless of how they actually received it. (VIP, Purchased, Transfer, etc.)
People started using this as a way to scam people, though. Since IMVU can't monitor user to user transactions like that outside of a 1 on 1 basis, they decided that it would no longer be allowed.
I'm not encouraging it, however, your situation is a half-legal way of doing it by having said person buy the nct from IMVU and have it sent to you as a gift. Giving them credits afterward can be considered a thank you. If you're gonna do it, might as well do it the safest way possible.
Advertising of buying/selling a nct can get your account disabled, remember that.