Am I out of line requesting a partial refund?

Hello everyone,

I've been buying camera gear off of eBay for about 5 years now. I've bought 3 bodies, countless lenses, and a few accessories. The other day, I received a Nikon F5 from a Japanese seller that was supposed to be in mint condition, but upon trying to load film into it, something was off and I consulted the user's manual and other F5 owners.

As it turns out, a part of the camera that helps keep film in place is broken, and as a result, I cannot load film into it. The seller quickly responded with "please give us a moment to check", when I reached out about this, but has not updated me in 3 days at this point.

The camera appears to be functional in every other way except for this defect, and fortunately, I can buy a replacement part very easily. I'd essentially have to replace the back panel of the camera, which easily comes off. I'd like to not go through the hassle of shipping it back to Japan, so the replacement part is appealing. However, this part is about $100 dollars (camera was about $500 total).

Would I be out of line in asking for a partial refund over this? Or should I just refund the whole camera? Part of me wants to refund the camera because there's no way it could have been validated to be functional with this part as it is. It's impossible to load film into it. On the other hand, buying the replacement part and asking the seller to cover it seems like a simple solution, but I don't know if it's realistic.

What would you do?