Welcome to Android Central! I don't think so. It'll cause headaches for the buyer. We hear about this with Sprint phones all the time--people buy a used Sprint phone, then find they can't activate it because the previous owner never settled their account.
If the phone is unlocked, yes. If the phone is unlocked BUT still not payed for, legally you shouldn't, and legally Sprint could blacklist the phone, meaning that while it might work initially, if they do, the phone won't work anymore.
B. Diddy and SpookDroid are right. If you don't pay up Sprint will likely blacklist the phone's IMEI number, turning the phone into a paperweight. They may have blacklisted it already.
You have to pay for what you buy -- always -- no matter what. So therefore you cannot sell it to someone else since Sprint will still want their money and they will blacklist the IMEI if they don't receive it. Which will basically make the phone a brick on all networks since they share the blacklist IMEI database.