To piggy back on, as this has happened to me as well.
Some buyers will try to get refunds just because they didn't want to spend money. So they may try to claim slightly damaged cards, refuse return shipping and demand a partial refund while using the feedback hostage. If I don't trust a buyer (or the item is a rarity or high valued) I'll put on insurance. Because any dings or lost items, I get reimbursed entirely for what I sold for.
The one time it happened to me a buyer claimed he never received because I only sent using insurance. Paypal sided with them and reimbursed. I filed a lost item report with the USPS through the military base I was at. Long story short, the buyer got hit hard with an investigation into mail fraud, paypal got dragged in for aiding without validation on arrival. Paypal refunded the money, e-Bay suspended the user (he's listed as no longer a member) and I got a nice personal apology from an e-Bay phone call.
The thing that use to kill me was in the Baltimore post offices, packages would get lost A LOT (I had 6 packages lost in the span of one month once using two different post offices) and I'd be out of money as the seller/trader. Fortunately, I knew this problem existed in Baltimore from losing personal letters so I always used insurance for packages over 10 dollars. This was before I gained my military clearances (I'm a civilian on loan to the military) to use and start sending through the USPS location on post.
Claude is absolutely right though. Baltimore is a rare example of what could go horrifically wrong with the USPS. I'd prefer DC anytime first but use insurance as a back-up only if I feel that I completely need it.