your usually better off not having private whois enabled if selling the domains as it leaves the option of direct contact available from buyers, You can always use escrow.com or moniker escrow to secure the funds if someone does contact you directly, the only reason I could see using private whois is if your holding adult or trademark names but all it takes is a legal notice and the registrar will release your contact info even if you are using private whois if doing something wrong, I have contacted quite a few domain owners directly in 2006 and wrapped up quite a few sales with the direct contact method which not only saves a 10% commission charge but also makes deals complete in 2-3 days instead of the 2-6 weeks at some aftermarkets, if looking to sell domains the easier you make it for a buyer to contact you the better.