in what way does Bitshares prevent this from happening?
decentralized solution to a centralized problem
But I doubt that if somehow BitShares features aren't included in one of those markets, that people will take the time to use it, unfortunately. But it seems there are a few decentralized markets trying to make it, so we should try our luck with them. I don't know the status on OpenBazaar though. SDC is apparently creating their own market, though I think it cant have illegal stuff.
A decentralized BitShares market would definetly make a huge impact, it would complement all of our features. The problem with this is there are already people developing markets so it kinda feels like a waste of time and resources, specially when we don't have much atm. Like we are reeventing the same stuff. But maybe down the road in a couple years we could have one! Or someone might just fork one
more on topic: people who use darkmarkets should know that's an accident waiting to happen. Either it gets shut down by devs or by the police. It's a matter of time really.