Auto Part that doesn't fit ...

[chariot_badges]

What do I do with a return because a used auto part doesn't fit?

Is that a ding on me?

[ebooksdiva]

You don't get a defect if your return is "doesn't fit" which is a remorse return. Unless you don't issue the label and the buyer asks eBay to step in, then you get a defect for cases closed without a seller resolution.
Otherwise it will show as a return in your dashboard return rate which is just a count of returns.