Certainly don't pay to have the thing annualed as others have said. Even if the seller pays for his mech to do an annual, you should pay for a detailed pre-buy because the person doing the work is working on behalf of the person paying the bill. The airplane needs to be in the shop of the person who is doing the pre-buy because you'll get a better inspection than if your mechanic has to travel away from his shop.
If possible, don't use the same mechanic for pre-buy that the seller has used for previous annuals because that mechanic can have a conflict of interest. People are just less likely to criticize their own work. Call it human nature.
Disclaimer: I'm not saying that the seller's mechanic would do anything unethical, but every time I have had an airplane looked at by a different mechanic, the new A&P has *always* found things that they didn't like that were either done or not done by those before. I liken it to proofreading (or like a dog marking its territory
- whatevs). Others will find problems that the author doesn't find in his own work. The new mechanic typically goes over the airplane with a squinted gaze and lets fewer things go unnoted (even if they aren't major problems) and that is why even if the seller pays for an annual, you should have a third party mechanic, paid by you, do a separate pre-buy.
I like the idea of moving the airplane to the third-party mechanic's shop even if it requires a ferry permit, because the mechanic works more quickly and can be more thorough at his own shop. The seller should pay for whatever it costs to get the airplane into flying condition - be that ferry permit or annual - that's not your problem IMHO. The lack of annual should be a burden on the seller, not the buyer.
You should expect to pay for the expense of actually flying the airplane to whomever you choose for the pre-buy. If the airplane doesn't pass pre-buy, it's
not your airplane. And thus it isn't your problem to get it home again. Offer the seller reasonable flight expenses to get his plane home but it's on him to get it done.
Who pays for what and who is responsible for what should be spelled out in the pre-purchase contract. Put it in writing and make both parties sign it.
As for the ferry permit, the inspection required for it is, in my experience, far far far from either a pre-buy or annual level of thoroughness. They just want to make sure the thing isn't likely to drop onto someone's house during
this flight. The permit I received involved a mechanic, 30-45 min of inspection to check flight controls, flight instruments, and engine run-up. A FSDO guy was present IIRC (this was like 1997) and issued the permit. The FAA may frown on multiple ferry permits, but again,
that is not your problem. The seller should ask the FSDO about that.
This is just my $0.001 from having purchased two aircraft in two very different ways. In the first, I used the seller's home-field mechanic (not ideal but it worked out OK). In the second, I paid to have the seller fly his plane to my mechanic. It didn't pass pre-buy and I paid flight expenses to send the seller home again. We later completed the sale after he had repairs done.