Are sellers more vulnerable to fraud than buyers when doing business?
My interpretation is more that, since sellers are the ones receiving money, they are the ones who stand to be protected by an improved and non-reversible payment system.  I don't think Satoshi particularly thought that protecting buyers (who are receiving goods) was less important or that they were less vulnerable, but that it simply wasn't within the scope of the problem he was trying to solve.  He thought that "routine escrow mechanisms" were already appropriate to the task, and in a decentralized currency, these mechanisms can be decoupled from the payment transfer system.