Because of the beautiful Aluminum and glass construction since 2007, seller and buyers thinking they are out of the box useful but quickly find out it is nearly useless. Here is why
Most iMacs (all the way up to 2019) shipped with a harddrive making them terribly slow and nearly useless with the newer macOS (operating system). Even newer macOS has obsolete browsers which can not use on many websites. This makes these iMacs generally useless for running macOS
All 2011 and older iMacs are stuck on macOS High Sierra with obsoleted browsers from all major browser makers. Making these iMacs nearly useless for running macOS.
All 2013 and older iMacs are stuck on Catalina which is already or near full obsolescence from major browser vendors suffering the same face as <= 2011
All 2007-2008s are stuck on macOS El Capitan. Basically useless obsoleted browsers with many unusable websites.
Fusion drives are terrible design. It doesn't help performance significantly and tje small SSD is worn out by now and drive system is near failure making the iMac useless.
Unless one is very skilled, iMacs are basiscally useless. Read below
Easiest solution on 2012+ is to use SSD on an external USB3 capable enclosure and install/boot macOS from this SSD. USB3 supports 5Gbits speed and internal drive interface is SATA3 (6Gbits) Therefore, no observable difference during use. Can acquire all the hardware and take it to Apple genious bar and I think they will help you do it for free.
Some people falsely believe internal SATA3 SSD will be faster than external USB3 dongle. Or if really want to put SSD internal to maintain the All-in-One iMac beauty, this require delicate operation to cut the glass LCD screen open to access HDD. It is easy to crack the screen during this effort even with all the special tools (plastic pizza cutting wheel) require complete adhesive removal which can damage the microphones, and finally without proper new adhesive (most amazon/ebay parts are junk) and adhesion primer, the retaped screen can even fall off! So be very cautious of all the youtube videos making it look easy (read the comments and you will see cracked and falling off screens). 27" is especially risky. Many of these are listed online with cracked screens as novice (and even experienced) crack the screen glass attempting this operation.
<= 2011s are easier (still not easy) but need solution to address Apple's customized HDD thermal sensor and ifferent solution for differnet year models requiring a bit of research.
After switched to SSD, can learn about OpenCore Legacy Patcher (OCLP) to intalled patched newer macOS on the iMac for full browser support.
This require learning how to install Windows via Apple's bootcamp feature. Also quite advanced. Windows will have newer modern browser support.
Very technical. Running newer macOS with supported browsers on these require GPU upgrade.
Summary is just to give it away to someone with upgrade expertise. Read below as its not a profitable effort but someone may be motivated to save these iMacs from recycling/trash fate.
Sell it cheap or give away to someone with advanced expertise to upgrade. Upgrades will cost upwards of $50-$100 and finished product is worth about $100. So its really a labor of love (usually takes 4-5 hours) to not see these things end up in trash rather than profitable enterprise.
Selling it cheap to unsuspecting buyers (don't know all above info since the machine looks beautiful) will usually end up in recycling/landfill eventually
Donating it will usually end up with unspecting buyers and end up in recycling/landfill eventually
Look through rest of this site for upgrade details. An additional guide ( link ) shows details on 2009-11 iMacs with GPU upgrades
2 key issues besides making sure its "unlocked"
While Macbooks do look beautiful, they do have design weakness areas that eventually end in its failure.
Battery will wear with age and often swell. Swelling can eventually cause false input on keyboard and trackpad. Not necessarily difficult to replace (more difficult ones are glued down) Challenge is to get high quality batteries as most aftermarkets are junk on ebay and Amazon (often marketed far more capacity than they really are) Apple OEM is prohibitively expensive. I purchase from mobilesentrix.com which is a professional repair shop parts supplier. You do need a business license to open an account with them.
All Macbooks will be referenced by the Model ID. These IDs are unambiguious references to specific models. Some Macs have 2 possible models in 1 year so best to clearly understand model ID. everymac.com is a good source to look through actual specs.
Just about all of these suffers from GPU failure. GPU's solder balls fail with thermal cycling. There is no cost effective repair. Some people "bake" the logic board in the oven (or use a heat gun which can destroy the logic board if not careful) to reflow the GPU. These "fixes" usually don't last but a few months at most.
In addition, they also inherit the flex-gate, butterfly keyboard issues on similar vintage 13" MBPs below
These are the bullet proof Macbooks (except for the worn battery issues above at over 10 years old now) DRAM is soldered down and not upgradable.
These were generally bullet proof (except for warn battery issues above at over 10 years old now) DRAM is soldered down and not upgradeable. Not Retina screen so doesn't look as sharp. Battery is not glued down and trivial to change but do need to source high quality/
Center of the LCD often have a 2.5" circular halo when viewed with constant brigher colors. This is from the LCD backlight sheet getting pinched with a circular mount for the apple logo when sandwiched tightly between books.
https://www.ifixit.com/Answers/View/256877/Why+do+I+have+a+circle+in+the+middle+of+my+screen
There were multiple problems depending on year. Common failures are
Flex-Gate : 16-17
Dust-Gate : 16-22
Butterfly Keyboard : 16? to 18
Touchbar Flickering
https://forums.macrumors.com/threads/macbook-dust-gate-preventive-cleaning.2439643/post-33476702
Probably avoid all of these
https://www.youtube.com/watch?v=7cNg_ifibCQ
A1466 and A1502. Downside is they are getting older and latest macOS is no longer supported and likely need to source high quality battery replacement.
OCLP can get many macbooks running latest macOS. However, there is a downside, newer macOS seems to cut battery life in 1/2! I suspect all post M1 macOSs (>= Ventura) are using more CPU more frequently as the cause.
Macs can be locked in various ways
iCloud
firmware password
Corporate issued
Here is a way to check all these
In case of older macs, just reinstalling the macOS will wipe out all accounts and passwords.
Many can do Network Recovery to reinstall the macOS
https://www.ifixit.com/Guide/How+to+Start+Up+a+Mac+in+Internet+Recovery+Mode/119437
If can not do Network Recovery, then need to do another way. There are variety of ways. My preferred (more advanced expertise required) way is
USB <-> SATA cable
2.5" SATA HDD/SSD with macOS El Capitan already installed. A macOS installed HDD/SSD will boot on any Macs that can run that macOS (For example, all Intel Macs macOS will boot on other Intel Macs which can accept that version of macOS)
Boot El Capitan from USB disk
Use free version of Carbon Copy Cloner or Super Duper! to copy El Capitan into the internal disk. Just reboot iMac afterwards
If want a virgin setup or El Capitan, need to download installer and install to internal disk. Requires manipulate dates to get to install properly