I can totally understand what you are saying here, and maybe I'm too trusting, but for those without PO Boxes, do you all freak out anytime a courier, salesperson, postie etc knocks on your door. 

If if I drop off something by hand, I certainly don't expect to come in, I just want to drop off and go. 

Ebay is based in a way on trust. Buyers trust sellers by paying a stranger in advance, giving their address, and expecting an item to be delivered that actually matches the description. I suppose I find it a little sad that everyone here is so paranoid about someone knocking on their door, or just leaving a parcel at their door. Let's face it, you have no idea who the postie or courier delivering to you is. Yes, bad things in life happen, and yes, be careful, but I know for myself I'd hate to live life stressing every time my doorbell rang.