If you are considering purchasing scuba diving gear that is second hand i.e. it is used scuba diving gear, then you need to take great care with your purchase.

If you are purchasing your scuba diving gear from a good friend or if the seller is well known to you or your local scuba club then purchasing the used equipment will pose little threat if the scuba diving gear has been maintained correctly.
For instance if you are purchasing important scuba gear second hand such as the regulator then you need to check the o-ring is not worn, has the protective cap been clamped into place after each use?
Has the regulator been rinsed with fresh water after each saltwater dive? Have you checked the hoses for signs of bulging or cracks?