Per oldacura's experience, you can take that a step further. You can involve a bike shop as the third party in a transaction. You can more easily find a good and bad shop via the forums, internet, googling and calling. I have done a few transactions where the shop is hired to give the bike a once over, pack and ship. The pack and ship part is not always something they are willing to do but you will at least have a shops signature on the condition of the bike. I have found the better shops more willing to do this. REI was even willing and had seperate member and non-member pricing for the service. A tandem savvy shop is preferred but no always available. Still more peace of mind than leap of faith trust in an unknown seller.