Then a reputable seller like KEH, Adorama, B&H, there are lots of them. last on my list is Ebay then Fred Miranda. There is absolutely no buyer protection on Fred Miranda, at least ebay will refund most of your purchase price if you get cheated.
Nearly all sellers on fredmiranda accept paypal. This gives you the same protection you would have if you used ebay (provided you use the non-gift option, of course.)
Not true!! He can send you a box of dirt, and as long as he has the signature confirmation, you are screwed. If I get a box of dirt using a credit card, my bank will chargeback, but PayPal does not cover you unless you purchase buyer protection.
Ebay will refund you if the item is substantially different from the description, but not PayPal. You can buy a moneyback guarantee from PayPal, but how many are aware that you have to pay to get a guarantee??
paypal have a "buyer protection" program that by my reading of the description does not require an additional fee (besides the standard paypal charge) I hadn't seen the "money back guarantee" thing you linked to before, so I can't really comment on it.
So no, you are not screwed if they send you a box of dirt. They do have a process that ends with you getting your money back in that scenario (in addition to the recourse your credit card provider offers)