Surprisingly enough, dark market sellers have excellent customer service. Since the market is completely transparent for a seller. On a number of dark markets both a seller and a buyer have a reputation that is tracked by the market. If a seller sees that you've bought before and this is the first time you ask for a refund they'll send you your order again.

Same with the reviews, a seller that has been around for a few years on a market, has good reviews, is most likely not a scammer. If they were they would be banned from the (reputable) dark market(s).

The fact that users' reputations are visible apparently lead to a widespread swindle called "selective scamming": sellers would ship as normal to buyers with an established reputation, but just outright rip off new users knowing that the buyer would be assumed to be the scammer.

But as a buyer to build a reputation I need to buy multiple things , this means I need to buy drugs from X, a gun from Y, some forbidden magazines from Z, my anonymity drops with each purchase. If I would do heavy illegal things I would not reuse same identity. I doubt that the mafia on the dark web have real reviews and fair escrows.