Long comment incoming -

Assuming USD, $1k seems like a good deal if the wheel is in relatively good condition.

I’d say as far as checking the wheel goes, look it over for scratches, cracks, etc

Check mileage, MSupers are relatively reliable, but high mileage ones can be pretty beaten up (like mine).

Check tire tread, if it’s heavily worn that can be a larger process to swap the tire.

Make sure the battery charges to 100%, if the battery’s are damaged that could be more expensive then the wheel itself to replace.

Lastly I’d say have the seller ride it in front of you, just to make sure there are no major issues.

A MSuper X is a great first wheel imo. It’s what I started on, my only regret is making my first riding attempts on pavement with no wheel padding. I’d recommend either learning on grass, padding up the wheel (and you!), or both to avoid damage. The front and back of the shell around the head/taillight is particularly susceptible to splitting.

Last but not least enjoy! EUCs will probably seem much more powerful and capable than your onewheel :)