Generally these sites have a system for establishing trust. So when choosing a hitman, you choose one that has already completed lots of hits. They could still

Generally these sites have a system for establishing trust. So when choosing a hitman, you choose one that has already completed lots of hits. They could still take your money and run, of course, but it's less likely. (So then how do you gain trust in the first place? Payment on delivery, combined with a trusted buyer.)