Not exactly sure what a “Lycoming wobble test is, but a lot of good points have been mentioned so far.
I tend to view an annual as an inspection that determines if the aircraft meets the “minimum” airworthiness requirements for continued operation. As a buyer, I don’t necessarily want a plane that just meets minimum requirements. I really want to know where I stand on things. A good prebuy also helps to decide if the condition actually matches the price. When I do a prebuy, I spend a LOT of time in the books researching hidden red flags, repetitive discrepancies, vague and unusual repairs, hose dates, ADs, etc. Of course I do all the common checks, like compression checks, and any other check the buyer and seller both agree on. I try to evaluate every item as an assessment to help the buyer make the right decision. For instance, I’ll look at even the simple stuff, like the battery. When’s the last time it was replaced? How long have the last 3 or 4 batteries lasted? How long before we should expect the current battery to last based on the historical data. Mags are always a hot topic, all the components are evaluated with the same detail considering time, hours, and condition. I’ve called prop shops, engine shop, and other specialty shops to help bridge the gaps between poor log entries and the actual work orders in an attempt to form a solid picture of everything. I try to build a timeline, including location of ownership, to help determine corrosive environments, hard flight training, and or other demanding situations the aircraft has been involved in. I usually conduct a full flight check to include flight characteristics, a functional assessment of all avionics, approaches, every switch, button, latch, knob, light, and all the other nit-picky stuff.
Ultimately, I’d say it’s more of a detailed conditional report of the current state of the airplane as it compares to the average, or the advertised description. You wouldn’t believe the stuff I’ve found on prebuys that were never caught on the annual, not required to be checked on annual, or were caught, but never properly corrected. Many times the uncorrected item meets the “minimum” requirement, and therefore the customer/owner never knows about about it, or the owner simply didn’t want to spend anymore money than absolutely necessary.
Yes, as mentioned, purchasing is a bit of a gamble regardless. Sometimes it works out completely fine. Other times a good prebuy can save massive amounts of money. I’ve found that a quality prebuy almost always identifies more than enough to cover the cost of the prebuy by a couple times over.