One owner, 45,000 clicks on the shutter, 2 64 gb XQD cards, 2 batteries, charger.......seems well kept.    Said he upgraded to the Z9.  

If you’re already at 3 grand for a well used d5 that came out in 2016, why not save a little more for the z9 yourself and get literally double the capability out of a new camera on 2021 technology? I know 2 grand is a big swing up but if you’re growing in your shooting maybe it’s best to sit on your current rig and save up?

If you’re really sold on a d5 that’s a fair price

I shoot/volunteer at our local high school, shooting mostly football games.    Currently have a Nikon 70-200mm f/2.8 FL lens and a Sigma 120-300mm f/2.8 lens with a D780, which I'm pretty happy with.  

Not sure how well those lenses would work on the Z9.   I have a Z6 and they work ok on it, but the Z6 is just absolute shit for sports.   Assuming the Z9 would be a bit better.  

vmax84: 45k clicks on a Nikon shutter is nothing, my ancient D40X has >150,000, and is still running perfectly today, the only problem is that there are no 3rd party batteries for it, only Nikon ones. BTW I no longer use the D40x today, instead I am using a Nikon D7500 & D500.

I keep kinda sniffing around for a good used D500 as well.     Not sure how the image quality would be for Friday night h.s. football games where you really have to push the ISO.

My understanding as of 4/16/2022, the D500 has been discontinued of the semiconductor shortage, but I have been wrong in the past, so who knows.

I have shot HS theater and stage with my D500, it focuses way faster & better than my Sony A6600, but unfortunately I can't use the D500 sometimes because of the noisy mechanism, the A6600 has a totally silent electronic mechanism, and even the mechanical shutter on my A6600 is pretty quiet against my D500.

I have never shot football pics, but I have shot marching band field show photos with the 1/250-1/300th of sec shutter speed on my D500 the image was respectable, I am also not making a big finished print either. I imagine that you would be in the realm of 1/400-1/500th shutter speeds for football to prevent blurring.

Personally, if it were me, I would just keep on using your D780 full-frame, it is a very capable camera. A couple of reasons for me personally for using the APS-C/DX/crop-frame camera is that everything is small and lighter, and to boot because of the small image circle lenses are way cheaper.