I do not understand how a plane can pass an annual before sale and then when the buyer has the next annual, there are many things that should have been addressed by the seller in the annual before the plane was bought.
I'll give an example of how it CAN happen.
1) New pilot/first-time buyer who knows nothing about buying airplanes, isn't a member of a type club yet, and has no local pilot friends yet to ask for guidance, finds aircraft for sale and goes to look at it. Seeing it's in rough shape he wants to rescue it from rotting away because no one has bought it yet.
2) Slick seller who offers said unknowing buyer an annual with purchase in place of a pre-buy inspection. "If there's anything wrong with it, it'll come out in the annual and I'll pay for it."
3) Annual being done by seller's mechanic, who yes, is familiar with the plane, but is also friends with the seller and also does the annual on the seller's Cherokee and wants to retain that business as well, so he pencil-whips said annual to get the plane out the door, paid for the annual, the seller some money, and continued business from the seller, and praise to other pilot owners from the seller for his great work an quick turnaround on the annual.
Sure, talk to the mechanic familiar with the plane, but at all costs, if you're serious, get a prebuy inspection done by an A&P that's never seen the plane before and will give a completely objective inspection.