Three days later you get a complaint from the buyer asking where the lens is. They deny having collected it and lodge a complaint with eBay. eBay will ask you as the seller for proof of sending (tracked parcel delivery). Since you cannot provide that, eBay will rule that the lens was paid for but never sent to the buyer and reverse the transaction giving the buyer a refund, such that your 'buyer' has obtained your lens for free.
Following the fraud that I suffered, I now only ever pay by PayPal and always send items I'm selling with full tracking. It's the only way to ensure that you are fully protected on eBay as both a buyer and seller. To stay safe, in my view, never accept variations on that formula and if the other party wants to do something different, always wonder why they would be happy to forgo their protection under the eBay guarantee. My advice would be to stay well clear of anything except a vanilla transaction.