Precautions buying second-hand mac + Clean install

Some of your questions would be answered by reading what the seller must do before selling a Mac (which include the fact that the seller must erase the hard drive and disassociate his Apple ID/password). That means you do not need nor would he give you his Apple ID. He must turn over the Mac with its original OS as the license for any updates or upgrades downloaded from the app store is not transferable and tied to his Apple ID.

What to do before selling or giving away your Mac - Apple Support

macOS versions and builds included with Mac computers - Apple Support

I can't help specifically with fusion drives - had one for only a week before returning the Mac and buying another with a regular drive.

As for the Applecare, I believe you will need to do that in writing (rather than online). Registering a new AC plan is different from one that is being transferred. You will need the Applecare registration number and a bill of sale containing all the info including serial number, etc., both seller and buyer names, addresses, etc., and a payment receipt.

And, since you are getting one year of Applecare, do check that the machine runs well, but if there are any hardware problems within a year, you should be covered. Just make sure that the seller did not open the iMac and do any repairs or upgrades (other than RAM) himself - if there is any damage, the warranty would be void.

If the seller disassociates the machine as he should, you should be able to use internet recovery, erase the hard drive from there and then simply download/install the OS.

If the machine still has an OS or app downloaded from the app store, the buyer will experience a lot of problems such as not being able to update apps or reinstall the OS, so it is good that the seller follow the necessary steps. The licensing rules and requirements have changed dramatically since they switched to download only.

As for the fusion drive: I can only relate what I experienced during the short time I had one. Fortunately, I made a fresh out of the box bootable clone. I partitioned the hard drive and later it was determined that the iMac should be considered DOA and I should return it and order another. Well, that is where the fun came in - there was no way to get the HD back to one partition; after many hours, the only thing that saved the day was that clone - I booted into it and was able to wipe the drive and clone the original setup back. So, I found a fusion drive to be far more difficult to deal with than a regular drive and I ordered another Mac with a regular drive.

I was willing to try it, but if it doesn't work, I will just buy an SSD and put it in an external thunderbolt enclosure, and run the system from there, using the internal drive as Time Machine backup or CCC.

That sounds like an excellent idea. I decided that it'll be either an SSD or a 7200 rpm drive (did you know that they now put 5400 rpm drives in the 21" iMacs???) if I can't afford the SSD. I can't see the advantage to simply boot up and/or launch an app a little faster while making it difficult to deal with - most of the real read/write/render video/images work is still being done on the HD.

Here's a Mac buying guide from Amazon that may help:

https://www.amazon.com/Buy-used-Macs-MacBooks-Checklist-ebook/dp/B00N01HYR8

I would add that a lot depends on where you plan to buy the MacBook. There are good deals to be found on eBay, for example, but you have to rely on the seller to write an accurate description and to thoroughly respond to your pre-sale questions. In my experience buying used Macs on Ebay I had one transaction go without a hitch and another that went South (read on if you want the rest of the cautionary tale ;-).

I had the experience only last month that an eBay store owner promised to verify/confirm the functions of a mid-2012 "classic" MacBook Pro prior to shipment but when I received the MacBook the superdrive kept spitting out my install media. Now I have been using Macs for a lot of years but every once in while I encounter a situation that is new on me and that's what happened: Turns out, in addition to ejecting the DVDs, the superdrive was etching my media with each attempt to read (to the point where some of my discs became unreadable). So it's stuff like this that you really have to be there in person to test as opposed to relying on the knowledge or integrity of the seller to confirm.

I bought my MBP from a highly-rated seller and because the seller said "Go ahead and repair it" (DIY replacement of the superdrive), Ebay wouldn't help when it was discovered to have additional problems upon opening it up (screws were missing around the superdrive suggesting someone had been into it before and an adjacent speaker connection was hanging by a thread and had to be re-soldered to the board by a certified Apple repair shop at added out-of-pocket cost).

EBay only ensures that buyers get their money back if the item in question is returned. If that item damages your personal property, as it did in my case, they are of no help whatsoever. The seller did nothing to comp me for damage to a newly-purchased Photoshop Elements and MS Office 2016. And now, nearly a month after purchase, I am over in another discussion trying to figure out why this same Mac constantly crashes my Pandora apps when it sleeps (incidentally, if anybody reading this can head over there and help, I'd appreciate it as the crickets are chirping pretty loudly :-(. So the number one precaution when buying a secondhand Mac is to know how the seller will to deal with the situation in the event of a problem for which a repair or return is necessary.

You will pay more buying used from MacSales, PowerMax, Gazelle or PowerON but in dealing with an established (non-eBay) seller, I think you are better off. In part, that's because buyers who open a Buyer Protection claim lose the option to leave feedback. So for eBay stores/sellers you will see feedback for buyers who had a good experience and maybe the occasional negative feedback left by a buyer who didn't ask eBay for any formal help to resolve a problem, but you will not see feedback for buyers who went through the formality of filing a Buyer Protection claim against the seller. In this respect, you can't trust the feedback system on Ebay to give you an accurate picture of how many satisfied vs. unsatisfied customers a seller actually has.

Excellent points. I'd add that roughly 90% of all software sold on ebay is pirated (that includes all the Macs which are being sold with app store OS upgrades); as well, I would not buy anything computer software or hardware related there after a (high rating) crook #1 sold me not the Aperture install media that I paid for, but emailed me an already often used and therefore worthless registration code and crook #2 sold me a "new/never opened" Mac Mini which was not only not functional, but absolutely filthy. Fortunately, I do not trust ebay either and only use my own credit card, so getting my money back was relatively easy: after submitting my proof, the bank simply notified ebay that they would do a chargeback unless there was a valid explanation. There wasn't.