It is a haven for scammers.
The admins enable them by being incompetent, complacent, or absent entirely, some combination of the three. If you go to the AlphaBay forums, you can find a thread where scam victims try to claim a refund. It is a completely chaotic cluster-fuck, and the best you can hope for is getting a reply from one of the staff there which will usually be something like "trying a partial refund." If you get your refund, I guess that is great, but if you don't, you're fucked. They probably won't even talk to you again. This is assuming you get their attention at all, which is unlikely.
I took a long hiatus from the DNM and placed my first return orders on AlphaBay. So far, my results are pretty harsh: 3 orders, 2 scams. Of course this is not really AlphaBay's fault, I just had a spell of bad luck and shitty vendors. However, it gave me a chance to experience the actual mechanisms which make AlphaBay a great place to scam people. Even if you are a proven scammer with dozens of people raging against you on the forums, you can probably still operate on AlphaBay ripping people off for a few more days before the admins step in to ban you. If you are a slightly more clever scammer, you could probably operate indefinitely under the radar of AlphaBay admins.
The problem is simple: AlphaBay has zero oversight. And really, if you think about it, all a DNM can offer is oversight and mediation between customers and vendors. That is the only valuable currency any of these marketplaces have to deal in. I am willing to accept the fact that this is the dark net, and there are risks to be taken, and by and large we are operating without any type of regulations or oversight. Yet you would expect the administrators of these marketplaces to provide some kind of support if for nothing else than for the sake of their markets. I'm not asking for touchy-feely admins who really care about my feelings, I'm just asking for someone who cares enough about their marketplace to want to put in the effort to make it more than just a place for scammers to advertise.
nobody likes alphabay. but it's going to work until it doesn't.