The other factor is that when buying a used vehicle you are also at the whims of whatever the previous owners (PO) neglected or whatever issues they had being fixed poorly. And sometimes it's stuff you'd never be able to tell until you open something up and are met with a nice surprise.
Example: My 96 jeep xj. The PO had a shop replace one of the rear axle/wheel bearings. My rear dif has a friction based locking mechanism that makes it so when one wheel spins too much it engages a set of clutch packs inside the differential carrier and locks it so both wheels spin in unison to create extra friction on the ground. When these clutches wear out you start to hear a kind of chirping sound, so naturally as they were wore out I decided to open up the differential and replace them....
When they reassembled the differential after replacing that wheel bearing, they failed to put any preload on the carrier. When the carrier doesn't have sufficient preload this can cause all kinds of problems because it can create excessive extra play between all the gears inside which can cause catastrophic gear failure. Even though the thing was a mess inside I was very fortunate the ring gear and pinion were still resuable. Spider gears were missing teeth and it was full of glitter and metal chunks. I had to overnight a set of new spider gears and replace the carrier bearings and redo the pinion bearings and crush sleeve. Here Is what the inner pinion bearing and race looked like and there is a picture of a chunk of one of the spider gear teeth and bits that were trapped between the bearing and race.
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With tools and parts needed to do the job I think I came out around $1k and it took me about 3-4 ten hour days to fix as I've never done it before. If I had a shop do it the price would have been so high that the only reasonable solution would have been to replace the entire axle with a junkyard one.
So I guess this really isn't the unskilled job everyone seems to think it is considering everything I've had to learn to stay employed...