Which firmware is it on, if it hasn't been updated that might be a good thing but you will likely need to download an older app.
Older firmware is/was open to owner modifications, mostly these are fine but some would push the limits to unsafe extremes.

Most owners have at least two batteries, you probably want three.
The app will be able to show how many cycles they have done.I wouldn't worry unless it's over 100 but in any case due to the smaller size they are not hideously expensive.

ND filters are OK for video but not needed for more casual use.
You want to see it flying and responding to inputs from the app on phone and controller.
If they claim some technical issue prevents a demo best walk away.

The mini SD card needs to be fast enough, try to get 32/64gb included if they have one.t shipped with 16GB that's a minimum.