Probably not the best expert on this - Ive only sold one second hand, not bought one (well, apart from the refurbished one I own now, but that was just about completely overhauled!) but the things I would ask would be:-
1. How old is the machine (obviously);
2. How often did you do a chemical backflush (should be probably at least every month or so);
3. How often did you descale it - this is particularly relevant in Perth where, apparently, the water is very hard and the scale clogs machines very quickly;
4. What water was used - ie; tap water, filtered water, etc (see above about hard water in WA - if they used filtered water, this should help the scale builld up so would probably not need to be descaled as often).
5. Have you ever had to have something looked at by a repairer and, if so, what was the outcome; and
6. If the machine is more than probably 2 years old, then ask what its service history is - I would imagine you would probably need to get a service done every year of two, but Im just guessing on this one (Ive not had the same machine for over 8 months, so far!)
I would have a good hard look at the machine and, in particular, pull the basket out of the portafilter - this should give you a good indication of cleaning regime: if its kacky with muck, its a pretty fair indication of not being cared for.
If none of the above are great answers, then at least bargain someone down in price if they are asking a premium. If theyre not prepared to budge when youre a bit dubious about the machine, then walk away.
Having said all that, if youre buying from a Snobber, then you should get good answers to all of the above, the machine should be clean and you should not have any problems at all.
These are just off the top of my head. I hope this helps - and, for all the others more experienced, please feel free to correct if Im being a bit hard or got something wrong.