(1) One argument for blotting serials: scammers using the pics for creating gun auction/for sale ads. Detailed ads with serials are more believable for buyers. The buyers can also run a check to see if the firearm was reported stolen.
(2) Another: someone might try to use your firearms information for insurance fraud. Not all places register all guns. Your firearm could be listed in a fire or theft claim for payment. Unless there is a need, Law Enforcement may never let you know your firearm was reported lost or stolen.
(3) someone might think they are doing a good thing getting guns confiscated by reporting your gun as stolen from them. In Hawaii, not much chance unless the gun was never registered.
(4) The main argument AGAINST blotting out serials was "you're paranoid! I've never heard of anyone ever doing any of this!"
1 & 2 are pretty specious. Manufacturers make lists of serial numbers available. It would be pretty easy to pluck a genuine serial number out for a specific make/model gun without having to wait around for someone to post one.
3 is just evil. But, yes, I could see that happening. It's the same mentality that reports private YouTube videos after being invited to see them.
4 is shortsighted. Not that many years ago, Social Security Numbers weren't considered particularly important. Hell, our own state used them as drivers' license numbers until recently! Just because there's not a widespread form of abuse today doesn't mean there won't be tomorrow.
If I am posting an item for sale, though, it's a little different. As you said, having a serial number lends legitimacy to the sale. In particular, with vintage guns, it's hard to claim "numbers matching" without posting the pics to prove it.