I bought an airplane about 12 years ago and had the pre-buy done by a mechanic other than the one that had maintained it in the past and that we continued to use. Here are some thoughts.
1)
Pre-buy should be done by a mechanic other than the one who has always done it in the past because new eyes see new things.
Not necessarily. All factors must be considered. Current mechanic may be the best choice.
2)
Pre-buy should not be an annual because annual requires a sign-off from an AI and pre-buy can be done by a anyone and doesn't require any sign-off because it isn't an FAA inspection. (cheaper)
Not necessarily, for two reasons:
a. If annual date is upcoming within a few months, or perhaps even longer, the cost of doing an annual inspection as a pre-buy may be cheaper than the cost of a pre-buy now and an annual later.
b. The inspection guide for an annual is (or should be) in writing and reasonably detailed as to the work performed. Many pre-buy inspections are neither. No guarantees either way, but at least some evidence if a "who shot John" scenario develops during or after the inspection.
3)
Pre-buy should not be an annual inspection because any discrepancies found will require that the aircraft be found unairworthy and the owner is then obligated to fix or abandon an aircraft that he intended to sell. Fixing becomes non-negotiable.
Not necessarily true. Some shops will try to use the pre-buy as a hammer, others will simply say it was in airworthy condition when it came in (according to the most-recent sign-off) so I'll provide both buyer and seller with a list of what we find and they can do whatever they want to with the airplane.
4)
Pre-buy only covers those things you specify and normally does not come with a cost estimate. You can say "I want a quickie." You can also specify you want all ADs checked, all cables inspected, every bolt examined.
Can you provide specific examples of the items you want checked and those you want omitted in a "quickie" as well as how you're sure that your definition matches up with the shop-owner's definition of such an inspection?
5)
Annual inspection is for airworthyness items and what needs to be fixed now. The pre-buy is aimed at finding what is beginning to wear out and will need to be fixed in the future.
Whose crystal ball will you use to forecast the extent and timing of future repairs? How far in the future does this prognostication reach?
6)
Pre-buy never finds everything. Annuals never find everything either.
Wanna see some invoices and work orders that prove this statement is incorrect?
7) Our annuals are never the same because Bill knows our airplane and knows how we use the plane. One year the headliner comes off and he inspects cables. The next year he examines the rudder checking for wear on all the hinges. You will use the plane differently from the previous owner, so your first annual will probably be more thorough. It is interesting to compare the pre-buy with the first annual after you own it.
Good luck. Have fun.