Hi Roman, I looked at that very same amp on eBay. I sent the sellers a message about the lack of serial number and they replied:
[color=#231F20][font=Helvetica, Arial, sans-serif]All items are fully tested by our engineers prior to listing. Regarding any serial no. or its absence, that's how this stock item came in to us.[/font][/color]
[font="Helvetica, Arial, sans-serif"][color="#231f20"]I decided not to buy because their reply suggested that they might be turning a blind eye, but I think you've done all you can.[/color][/font]