Never Post A Wanted Ad ?
While true that never posting a wanted ad will limit your exposure it will also limit your potential ability to obtain the parts you covet. Most of my Purchases and Sales have been a result of responses to a member's needs.
While "Ply30" is an exception to a long time member gone bad I would certainly think that dealing with tenured member's in general has less risk. Not to say that somebody that joined five minutes ago is necessarily going to rip you off it's just that there is less of a track record.
As always in life: Caveat Emptor