Some fair points made re second hand stuff, but......where does that leave club kit? I mean, most of us will hang off club ropes and maillons/krabs, or use club SRT stuff and that is, in effect, second hand. Indeed, it might well be more 'second-hand' than someone's personal kit.
When I climb with other folk I may well use their kit, and trust my life to it, but I have no real idea how it has been treated or stored. I did once 'drop' a mate's knackered sling in order to get rid of it [and gave him one of my newish ones in return] but generally we all use each others 'second hand stuff without any drama.
I think a bit of common sense will see you right.