P2P transaction would be the safest but we know its only limited via localle but if you do tend to go international then having escrow would be suggested if you are buying into someone.
Its a hassle process yet trust would be the main issue in most cases because due to anonymity feature of crypto then someone can easily scam out without delivering the item or
simply a fraud.Theres a dedicated board for buy and selling of goods on this forum which you can actually check it out.  https://bitcointalk.org/index.php?board=51.0