Has anyone here used the ebay alternative Mercari? It truly is a wild west where you have very little buyer protection, and scams run rife. It's important for buyer protection to be highly prioritized on ebay so that people feel comfortable to purchase items. I've never had an issue over the past 10 years buying parts on ebay. I think using ebay is a combination of personal responsibility to eat costs where you didn't do your due diligence regarding the item's condition/compatibility, and utilizing the buyer protection inherent in ebay's system for those moments when the seller did lie about an objects condition or is unwilling to play ball and compromise if the item does not meet stated expectations.

That being said, I pretty much never trust a seller to know a bicycle part's measurement standards/compatibility, and only go by the markings available on the packaging or the item itself. If I'm ever unsure, my questions get directed to this forum. Returning NOS is difficult...I think it's fair that the seller would not refund 100% of the cost, but trying to keep 50% is really scummy (assuming you did not mar the part or packaging). Unfortunate that you had to deal with such a dishonest person on ebay, but them's the brakes. Hopefully you playing hardball with the refund will incentivize them to change for the better.