EDIT: I should've rephrased the question to ask if anyone is or has ever been aware of a specific (unnamed, that's policy, right?) vendor doing this. and if anyone has any good stories.
There is one vendor (a 'trusted vendor') on Oasis that I was curious about. They had a significantly higher score than anyone else. So I tvake a look at their reviews and notice a trend/
They put items up for sale at virtually no cost and, using fake vendor accounts, give themselves positive feedback. Much of the feedback is so similar it's funny. They then turn around and make the listings a believable price.
This person has a score on grams of over 1000.
I'm willing to bet a good amount of them do.