How do you come up with that?  You may not khold back more seed then you will use.  You can sell leftovers, but that is kind of a stickly point because someone has to define why it is leftover, so best to just skip that.  Trading with a neighbor and using what you traded for indicates that you did not save more then you could use, and if he uses what he traded for, neither did he.  I do not see this running afoul of PVP.

Your mileage may vary.