I'm afraid that idea was considered whilst discussing the site updates quite some time ago.
Even without posting access, it would still allow scammers etc the option to view the sales & arrange a deal via pm.
We discourage dealing via pm & insist all deals are posted on the actual sales thread. All details such as payment details & addresses should be kept to pm's.
If it is all done via pm it essentially becomes one members word against another whereas if the deal is conducted in the sales area it makes it easier for us to track should there be a problem.
We pride ourselves on there being very few bad deals conducted here & these rules have helped keep it that way.