The Times They Are a-Changin'
Apple now publishes detailed repair manuals for iOS devices and soon Apple Silicon Macs will have these documents widely available. Apple will sell parts and rent tools as well for DIY repairs.
You can still use authorized repair partners, first party AppleCare repair staff or choose anyone else you trust to do the work.
Some will revel in the right to repair, learn and perhaps start a career. Others will discover the right to despair, when they get in over their head or break some part that even trained technicians will break (but the good ones learn fast and make up for extra part costs early in their career by doing many profitable repairs over their span of being a professional fix it person).
Apple resellers are allowed to both perform repairs where the broken part goes back to Apple and just sell you a part. It’s not clear yet how the DIY parts pricing will encourage or discourage others from reselling parts. I believe their costs are way higher for the costs of an "one way part" so in practice, this does not happen much on major components that are not commodity parts.
In your particular instance, I would look to see when that specific model goes vintage. Presumably, Apple would then sell their stock of parts to the repair centers and for a limited time, there might be a nice supply (prices low) and no risk to the people carrying the part that their costs to carry inventory will raise the price of that part.
Even from reputable sellers, many parts that exist on the open market are scavenged from machines that are damaged (sometimes by liquid) and disassembled for parts and carry only a short warranty if any from the seller.
This is a good thing in almost all cases - reusing perfectly fine parts when they are properly tested is inexpensive and proper. The flip side, is what machines get damaged and don't get fixed? The worst kind of damage.
An iMac is very much not at risk from liquid but other things like power surge, can affect parts in an invisible way. Again, the question is of self-selecting. Which macs don't get cheap parts to fix the issues they have? The ones with major problems to either the display or the logic board.
For these reasons, I prefer to buy parts from a company I trust to know why a used mac isn't getting fixed so I can get good parts from them. Similarly, I'll gladly buy the damaged mac from the person that wants to sell it for parts so I can evaluate the damage myself. They are usually forthcoming why and how it got damaged ( and also pricing my offer knowing it may be a total waste.)
As always, finding a good mechanic - someone who knows the business and has experience with hundreds of repairs on your specific model and thousands of repairs under the belt is more important to knowing when parts can be used.
For the last two decades, specialized and highly reliable parts outside of Apple's supply chain are too expensive to be widely available. It used to be hard for consumers to know if a good price is a lucky break or the seller has more information of the part's heritage and passing on hidden risk to the buyer. Hopefully these new DIY options help everyone make good choices and repair their gear for as long as it can be serviceable.
