The simplest solution, IMO, is to buy only from sellers whose reputations are sufficient to justify trusting them.
Interesting point, Ebay had exactly these problems when they first started and have solved the problem already through a rating system (although that can be scammed with fake accounts)
So what about a change to bitcoin that allows for a confidence rating to be associated with an address, or is this a development opportunity for a third party website to do the same, where you paste the address to get a confidence rating. Merchant's might use the same holding address to build confidence for their business and customers might take the time to provide feedback if it is simple and are keen to improve the community.