If you don't have the expertese to properly check a used mower for faults then you will be best served to buy one from a used machinery dealer that has a warranty.
PArticularly one that sells real tractors or directly from a mowing contractor / golf course / hospital / municipal authority .
Buying privately is always subject to the ethics & morals of the vendor and there are way too many totally dishonest sub human people out there .
And feel free to come here and ask what you should be looking at for weak spots or potential trouble points before you close another deal.
There is a lot of techs & private owners here and between all of them some one should be able to point you in the right direction.
If you choose to do this please let us know what you are mowing , some photos are always a good idea .