If a grocery store harms you in an illegal way you they can be punished by the force of the state in a variety of ways. The same is not true of a anonymous person on an inbound tor site.
If someone sold bad shit on the silk road, they'd lose all of their rep and stop making money.
On the original silk road, it cost a very large amount to get a vendor account. So if you decided to start selling bad shit, you'd have to play nice for a while to break even, a while longer to build trust, and then go out in a blaze of bad shit.
I don't really see why anyone would do that. If you have good enough connections to get a good rep on the Silk Road (or similar site), you have good enough connections to never need to sell bad shit. As Stringer Bell would say, it's all about the product. If you have better product than your competition, you'll win in the end.
This is more recourse than you have in practice against a grocery store if they refuse to give you a refund - what are you going to do, sue them?