[...] A Cessna service center is probably the best place to do a pre-purchase as they follow the standards of the manufacturer. [...]
We though the same, when we bought our plane just a few weeks ago and paid almost exactly $1,000 for a pre-buy inspection at a Cessna center. The pre-buy included an IFR check. Of the options we had in this area, this was the most expensive one, but we wanted to have it done right and rather pay a little bit extra. Well, this was pretty much a waste of money. They even missed a number of obvious and quite common defects like a completely worn torque link at the front gear, a broken spring at the seat lock, which would then just slide back in the worst moment, and an inop compass.
@ Bryan: What I want to say with this is that if somebody has a least a some technical understanding and is interested in this topic, one should try to become as smart as possible and participate in the pre-buy inspection, instead of just kicking tires, maybe flying the plane a bit and send somebody for the pre-buy either later on or in advance. I have also learned that it is more important to send somebody you trust rather than to just select a company because they have a good sounding name and are e. g. a Cessna Center.
Another reason why I believe that it is important to participate in the pre-buy is that having a second pair of eyes never hurts, even if they are untrained and that one can directly see whether an issue is acceptable or not or if something would be seen by the potential buyer as an issue, what the guy who does does pre-buy might not even consider worth mentioning (we had both).
Also, speak with other pilots, A&Ps, read the forums, ask question - reflect on their opinions, take everything with a grain of salt (easier said than done, I again speak of own experience) and put it into context, try to make up your own mind and turn it into a learning experience. I was really surprised (well, maybe not) how greatly the opinions varied with the tendency towards more radical position in internet forums.
Speaking of seeking advice - I learned, especially from researching the internet, that one must insist (OK - I am a exaggerating, but only slightly) on hundreds of detailed pictures, scanned logbooks, multiple discussions with the sellers mechanic and the commitment of the seller to fly the plane to another airports, possibly hours away, for an extensive pre-buy inspection. Of course, the plane must also be in a like new condition to be acceptable and will always be too expensive, no matter for what price it is offered. It is also important that it is not too far away, but lives in a dry region. Yadayadayada...
While these are certainly great tips, I believe that they describe rather an ideal than a real world process. At least not, if the goal is to eventually buy a plane in the $25k - $75k range.
My experience, from searching for multiple month for a 172 for up to $50k and contacting close to 30 sellers is, that quite a few don't even respond to e-mails or calls, even though they just put the plane on Barnstormers, Trade-a-plane or wherever. Of those approximately 20 with whom I got in contact with, only one(!!) was able to at least provide me with reasonably good quality photos, a video of the plane and who tried his best to answer my questions. This was the dealer from whom we actually bought the plane. He was also one of only two who agreed to bring the plane to another airport for a pre-buy inspection. To scan the logbooks was beyond anybody was willing to do.
I also found that the planes which seemed to be really good for the price, were often gone pretty much instantly. Quite a few were already purchased on the phone, with the buyer on its way, by the time I called a few days, sometimes hours after they were put on the market. Even when planes were just 'normally' attractive, they still created enough interest that sellers did not feel that they would need to go through all the hazzle with snapping additional pictures, scanning logbooks, arranging calls with their mechanics or to agree to a pre-by inspection a few weeks later, because there were usually a number of potential buyers aligned who would show up within the next few days.
OK, this was a long post - I learned that it is essential to become smart myself about what to look for and to be prepared to go to check out a plane on short notice, if it sounds right on the phone and if it looks good on the few pictures which are usually only available. To be able to conduct a pre-buy without another person also wouldn't hurt and increase the flexibility. My experience is, that the good planes don't stay long enough on the market and the sellers don't feel the pressure for a full blown ideal purchasing process.