You need...absolutely NEED...to have supporter status, which requires a $20 acount upgrade ($200 for annual) to post either buy or sell adverts in the PX. You can respond to any posted advert without needing supporter status.
And, all such adverts threads must be created in the PX (which you can't do without supporter status) and no where else.
Breaking these rules will indeed cause cause ire in the mods and if ignored then a time out may be issues.
Proceed at your own risk.