I don't agree on point (3)... if the sellers fakes the autograph he surely will not mind faking the letter of provenance, in fact most people who fake items tend to add a COA. That piece of paper is worth absolutely nothing to me.
Anyways, just my opinion.
Anyways, just my opinion.
(3) If the seller won't provide a letter of provenance OR such a letter does not add up, avoid the item. Such a letter is all important to my mind. If it's eBay, check out the seller's feedback and ask the experts here about the item. OK, you will alert others here to the item. But I'd bet that "everyone" already is aware of it. It is, after all, eBay!
Beren as expected is correct here, you need a trustworthy independent source of provenance for the item not a COA from the seller, a letter of provenance only means something if it can be reliably and independently validated.