The correct solution to this is a 3rd party escrow service for these expensive items prone to fraud.
Amazon could physically validate the lens with pictures/weight, and verify they shipped it directly. This blocks fraud by both the customer and the seller/shipper.
If I'm buying something off of Ebay from a private seller (particularly used gear and such) that is going to cost more than I feel I can possibly comfortably lose ($200.00 is the upper end there), then I ask the seller first (before doing a buy-it-now or bidding) if they'll do escrow (usually via escrow.com, since that used to be an ebay thing).
If they won't, I don't bid or buy - because it is likely a scam of some sort. Usually, I explain that the protection is both for seller and buyer, and that I will pay all fees involved (so they aren't out anything). If they still balk, then forget it. Not worth it, no way.
I've never used Amazon Marketplace to buy used goods or anything like that (everything I buy from Amazon is prime stuff; rarely do I do third-party seller unless its from a place that has a real website too - and even then I exercise caution).
For large cash transactions via Craigslist? Take a friend with a gun, in case things don't go the way they should. My last purchase from CL was a car, and I swear I thought at one point I was going to be rolled for cash I didn't have with me. It was in the vehicle my friend was driving; I was riding with the owner in the vehicle being sold - we were trying to find a place to transfer the title - I know that doesn't sound like it should be difficult, but the story is much more complex. In the end everything went fine, the guy got the cash, I got the car, and we parted amicably.
Amazon could physically validate the lens with pictures/weight, and verify they shipped it directly. This blocks fraud by both the customer and the seller/shipper.