One question here... Is there a way to have a secure pin or do people who buy pins in bulk just have to trust the other person?
This is a thread better suited for the blackmarket lounge but I'll answer your concern anyway. If by security you mean the seller charging the pin back, it can always pose a risk. There isn't such thing as a safe pin as they can always be charged back. However, instead of trying to buy a pin you can try to purchase an RS membership card off someone. I like to buy the cards because it's safer. It can't possibly put your account into negative credit unless it was stolen and you happen to be caught.
Dont buy bulk pins unless you get the pins first, sell them, and then give the guy the money. Dont ever buy off people that dont have a rep of previously selling legit pins anyways.