Selling dispute. Please comment.

Your positive feedback indicates at least 44 hassle free transactions dating from 2009. Unfortunately, representing that the original manual was included with your amp sale was a mistake - we all make them. You had the opportunity to verify the manual was present before listing the amps, and again before shipping them. Having the original manual is apparently a big deal to your buyer. If you cannot supply the original manual, or otherwise satisfy your buyer, let them return the amps (you pay shipping) and start over. Anything less and you have an unhappy buyer.

Like most things in life, you play your own hand. At this point, certain facts have occurred and this really boils down to your desired outcome, which you can largely influence.
On one hand, you have your money, the buyer has the amps and a "copy" of the manual, and you can simply say "enough," or "piss off," - your choice. As Lloydc indicates, you substantially performed and damages are likely limited to the potential loss of resale value due to the missing "original" (vs. copied) manual - e.g. small. The likely outcome with this solution is that you will receive negative feedback and the buyer will not bother taking you to court over a stinkin' owner's manual.
On the other hand, you could ask the buyer "how can we resolve this so that you are satisfied and we simply forget feedback?" Be ready for requests ranging from a partial refund to return of the amps and full refund. This outcome probably results in some level of hassle and pain to your wallet, but does not affect your feedback, assuming the buyer agrees.
The cards are dealt, it really boils down to how you want to play it.

Shoff, you make some good points, but I would add that if this were Wyoming or Montana (or a lot of other places), the seller would have worked this out with the buyer and been done with it without subjecting the whole pitiful experience to the court of public opinion.

IMO, the real problem is that both buyer and seller would rather throw nuts at each other than own up and move on. Jaxwired's original talk about

various papers from the manufacturer in the boxes

and especially his more recent post

So when I checked the "manual included" box I really meant was that the buyer would get whatever paper docs were originally in the box. Not so much that they would specifically be a manual.

is absolute BS, and simply a justification to himself (and to us) that he didn't "actually" screw up by not verifying the original manual was in the box before listing the ad or shipping the amps. I get it, we all screw up sometimes, but quit whining about it and resolve it.

A review of the Audiogon "Seller's Checklist" shows "Do you have the owner's manual?" being one of the questions to answer. In this case the seller did not, but checked that he did. Regardless that 99 out of 100 of us would have happily accepted the pdf manual, and regardless that the buyer comes off as an ass-wipe from his e-mails, the fact remains that the seller did not complete his commitment. Therefore, this boils down to either the buyer accepting the proposed solution (pdf manual), or the seller accepting the return of the amps.

BTW, when you post an ad, the seller's checkbox reads "Owner's Manual Included" but the actual ad lists the manual under "Original Accessories." The absence of the word "original" in the ad posting checkbox and the inclusion of "original" in the listing is a discrepancy that Audiogon should fix, especially after this thread.