Also, RobertBJ, you seem to be hung up on the fact the OP listed it as NEW. He didn't. It was NEW (OTHER) which directs you to the description for details. The OP says the item is mint, original box, etc.
Basically, he bought it, hooked it up, tried it out and then Siglent released a newer model, which he bought to replace it with. The NEW (OTHER) category is often used for display and demo models. There's very little difference between that and what the OP described.
Honestly, I might have listed the item the same way. If you read eBay's guidelines for the NEW (OTHER) category, he's well within his rights. Keep in mind also that eBay doesn't have rules. They have guidelines. Also, eBay employees tend to make shit up as they go along, often contradicting themselves from case to case and employee to employee. I have personally witnessed this, as have family members and people on this forum.
Finally, you're not going to win this fight because you're wrong. How wrong? As wrong as Siglent was. This wrong: