Hum sound ..is it me or is it you? LOUD hum in system

got up this morning took the day off and played some more with it.  no hum but I do have damage to it on the top front face that was not there when I shipped it out.    it wasnt shipping damage as it was wrapped up tight and secure both ways.

in this whole ordeal the only thing that guy did right was pack it back up.

im still very angry  over this nonsense

Sold a hi end headphone amp and guy says "buzz like a refrigerator".
Figured ground loop, but can't know if maybe shipping damage so took it back. Quiet as could be. Resold it and new buyer says dead quiet. So I got stuck. Cost me $200 in PayPal and shipping. :/
Spoke with the manufacturer in the middle of the process and he says gets a call every 2 weeks from someone complaining of noise and takes the return to find the product is virtually always fine.

hi
From a buyers point of view, i purchased a pair of Aragon Palladiums,
From the high End broker in Holland via paypal 2300euro
They arrived faulty and had been sent from Germany not Holland, one of the amps had low output with distortion,
After providing an engineers report and witness statements to paypal proving my case beyond any reasonable doubt , i had my paypal account closed,
When i offered video evidence which i sent to the seller, paypal insisted they do not except video evidence, so in my experience as a buyer via paypal taking a video of the amplifier was not excepted so i doubt that would help a seller either,
Anyway 6 months later im stuck with the amps they get repaired then go faulty again,
A dealer took them off my hands for 1000gbp so i ended up over a 1000gbp out of pocket, lots of stress and involving third partys for written statements to paypal,
When i answered every question and proved my case beyond any reasonable doubt , they closed my account, leaving a balance of around 1500gbp,
Which i had to wait a month or 2 for but was told they could hold it up to i think 6 months or a year,
I would never buy from High End broker again he is just a platform and takes the sellers word things work,
And i would never buy or sell anything using paypal again they are a scam without question.

I completely understand your frustrations and believe that unless you expressly stated that your item could be returned then you are not obligated to do so. My understanding is that used equipment is as is with no return.
Most importantly you have no control over how the buyer connects all of his associated equipment.
The preamplifier should never have it's grounded AC plug floated with a cheater plug! Everything else is fair game for this scenario and is actually the best methodology to eliminate hum.
Multiple AC outlets can be a primary cause of ground loops and consequential hum. And you sir are not responsible for the buyers poor or incomplete understanding of the proper procedure to eliminate hum.
I would advise you to communicate this to the buyer and that the solution to correct the hum issue is not inherently the fault of the Classe preamplifier he aquired from you.