The CCP rules are that you must be in an NPC corp prior to posting your thread, and throughout the sales duration, and not as to how you’re doing it.
As per Welcome to the Character Bazaar - Marketplace / Character Bazaar - EVE Online Forums (CCP’s rules on character sales) I highlight for you:
B. Before making your thread
- The character being sold must be in an NPC corporation at the time of posting, and for the entire duration of the sale/auction thread being active.