First, I am very sorry you had to suffer this encounter with random violence. Something similar happened to one of my kids recently.
As situations go, carjacking doesn't have much in common with selling audio, other than the fact that some people will want what you have and might shoot you while taking it.
Where I grew up, a paranoid mindset could help you go through your childhood and teenage years relatively unscathed. But the circumstances warranted it, whereas defense mechanisms activated in the absence of credible threats are a hindrance and a liability.
Folks who demand copies of buyers' drivers licenses and other nonsense (assuming they're actually serious) are driving good buyers away. Unless they're selling their stuff at such a discount that it makes sense to jump through the hoops, however ridiculous those might be.
As far as guns, even if one is carrying it doesn't make much sense to pull a piece on three armed guys unless one is a really, really, really good shooter. So there's that.
I don't think selling audio is a particularly high-risk activity, which doesn't mean it's absolutely completely 100% risk-free. "Trust but verify" is always a pretty good attitude.