Maybe. But, doesn't it make sense for a vendor to spend 100 BTC in fees to build a reputation so that they may scam 1,000 people in 1 BTC transactions?
Yes, it does. Which is why you have to have the trust factor incorporate the transaction volume for each vendor (calculated based on how much they've paid in fees).
Scamming 1,000 people for 1 BTC would require having 1,000 open orders for 1 BTC each at the same time. If a vendor who has paid 100 BTC in fees has over one hundred 1 BTC pending orders, you'd probably be wise to stay away. And the software can guide you in making this decision.
Allow me to disagree (with the last sentence). If you can see that a certain vendor has spent a total of, say, 100 BTC on fees, to build up a reputation, then it doesn't make financial sense for that person to try to scam you in a 1 BTC transaction.