I don't think it would be an easy thing to do.
Idea number 1:
Keep track of his PRIVATE auction winners from his feedback section and see if any of them use his documents as proof of a cars history to sell a car on ebay. Bust a few of them for fraud and maybe put a crimp in his business if he got drug into court a few times.
Idea number 2:
Not legal and can't be listed for public viewing.
Anyone up north know what I mean, spring thaw and the rivers are fast making things hard to find? LOL
To be serious, no matter how good he is a fake document can be detected.
1)Ink compositions are different and the text will exhibit different characteristics if printed with modern ink.
(Problem would be if he is using a 40 year old typewriter that still had a good ribbon)
2)Paper fiber composition is differnt now due to use of recycled paper. Recycled fibers behave differently the second time around due to the second bleaching/cleaning process. Atypical fibers in the "aged" areas could be a clue to a faked document.
3)If he is making them with an inkjet printer, that can be spotted under magnification. The type will have a tiny spatter pattern around each letter that the original imprint type would not. I think this can be seen with a 50X or 100X loupe.
4)If you want to be 100% certain about a car you are going to buy, clause the deal with an authenticity writer.
Find a local test lab, such as A-Labs, and have a nondestructive age test done on the docs. Most often it would be a light or vapor/gas test of the ink or paper. Based on other testing that A-Labs has done for me in the past, I would guess the fee would be less than $500.
I will never look at a cars documentation the same trusting way again.
OK I'll climb down off the soapbox now.
But as the saying goes
"If it sounds too good to be true, it is"
or something along those lines.
Later,
Eric