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I just received an email from ebay, to a gmail account that I don't use for ebay. I sent the email to ebay who verified that it did not come from them. However, DKIM says it is a valid and genuine email from ebay.
Not sure if DKIM has any other use than the simple one I'm using it for, but for me it's a massive fail.

Could you please send the e-mail as a saved eml file to me via e-mail (lieser+dkim@posteo.net)?

I would be very interested to better understand what is going on, instead of guessing and explanation.