Centralized Online Systems
When buying from someone on Amazon you see a rating system and you also see comments on products. Amazon can use high level statistics to find abuses of its system. Also they may use other techniques and closed-door policies to screen out abuses. Identity can be linked with real people through credit card, address and phone information.
Trust Problems on Decentralized Systems
OpenBazaar is facing a similar problem, except that much information is public and users are often anonymous. As a decentralized platform the mathematics behind the selling and credit system are open for abusers to see and use. Anyone anywhere can pretend to be multiple users, who can then up-vote their own content and pretend to be highly rated.
How do we solve this problem? How can we trust sellers who can hide from the law? How do we trust buyers who may not even be real?
Current Methods of Trust
First of all, people already have methods of discerning scams. If one sees a commercial on TV, he can assumed that it is biased but probably not false-advertising. One can assume that well known companies are being kept mostly honest by society. In general, one begins to have a feel for those who are trustworthy.
The Ideal Solution
I believe the trust problem OpenBazaar is facing and trying to solve, has a solution. This may be to simply verify each persons identity. A decentralized platform can allow third-parties to verify real-world identity. If big enough, these third-parties will be held accountable by society and law. We already verify identity in banks and other places.
Once we know full-users are real people with limited accounts, we can rely on all the same systems and methods that have evolved through time.
We can still protect identity from general public knowledge, like whois for domain names. But yes, this actually makes users not fully anonymous.