What responsibility does a seller have to tell a customer an item has been serviced?

A few months ago I bought an expensive pre-amp from a dealer here on audiogon. This is a long time known dealer with an 100% approval rating.

I went to his home to listen to it and there was one quirk about the design I didn't like about it but I thought I could live with it. I had spent so much time and effort to get it. A pretty long drive. When I brought it home and within 24 hours by the afternoon of the next day I realized I couldn't live with it I called him to return it. 
He didn't want to take it back. On his website he has a 30 day return policy. Unfortunately not being an experienced buyer I did not know there is a hidden tab on audiogon saying no returns. Why is this not stated on the main page?!

I was furious and he said he would take it back if I forced him to but he already had another buyer for it. I felt bad and there was a (hidden) no return policy so I let it go.
I had given to him two excellent pieces, the pre and an integrated, and cash for it so I lost quite a bit of money on the deal. I'm obviously not an experienced savvy customer.
I asked him to just give me the pre back for the piece he sold me and keep the rest but no deal. And he told me the pre I gave him in perfect condition, the one I wanted back, wasn't working so he had to send it in for repair. But I shouldn't worry about it. What?!!! I drove it to him I didn't ship it! And why shouldn't I worry that I gave him a non working component when I told him it was in perfect condition?!

Then about a month later I saw he had it listed for sale. This was about 2 AM and I immediately wrote him asking for it in exchange for the item I bought being that the sale he had fell through. Early the next morning hours later he told me it was sold and he had taken it down.

He did list my item he sold to me on audiogon for a couple of months at his expense but no sale. I finally at an extreme loss as I said was able to use it for credit towards another item being sold by another audiogon dealer. This dealer told me he spoke with the American distributor of the brand who told him the first dealer had sent it to him for repair. This was not told to me. Of course I was outraged. But I hadn't asked. If I had known I would never have bought it.

So this is my question: What obligation does a seller on audiogon have to tell a client an item has been serviced if a dumb customer doesn't ask?

Still burned by the loss of so much money and the jerking around and what seems to be a couple of lies.

If the item is an older unit, I would prefer a professional open up the hood and take a look at least once. The sellers that irk me are the ones that claim their "tech" recently serviced the unit. When pressed for proof of work done, sellers rarely can cough up any information of any kind. Saying the unit was serviced and not having any kind of proof? Yeah, thanks but no.
'Anyway he is not obligated but it would have been a nice gesture.'
Yes and as I said he DID make the offer. But being that he told me he already had a customer for it I felt it incredibly unfair to take a sale from him when he kept his part of the bargain.

When I found out the deal fell through I decided to take him up on it. You know what happened next.

Then of course finding out from the guy I gave it too that it had been a serviced component was pretty embarrassing and it got my goat. Remember I was not being transparent with the guy I traded it to. I'm guilty of what the seller did simply because I did not do my homework. Just kicking myself that's all and wondering do you offer important information that can hurt a sale if people don't ask? Or do you keep quiet and hope you have a dumbo.
No, you ask questions next time around !!!  Caveat Emptor!  Before you drive all the way there, and trade him 2 of your units + cash for his unit, you ask questions, you find out about the gear you are acquiring!  Please stop driveling on about this situation!  
Honesty is the best policy - Full disclosure of all known faults to the best of your knowledge even if you sell it "As-is.  Where-is".  Protect your personal reputation, avoid possible legal claims, especially in this day when people complain about you on social media....and they are telling the truth.
Elizabeth’s suggestion of a plaque to commemorate this horrible transaction is hilarious (LOL)!