You're right about the red-tape in general, but the reality of unlicensed work isn't nearly that extreme, at least in the US.
First, there's quite a lot of unlicensed work that is entirely legal in many places, and a lot of it is fairly common work -- replacing or repairing fixtures, for instance.
Second, there's no "list of unlicensed work" either -- sellers are required to disclose unpermitted work that they're aware of, but as you can imagine, compliance here is sometimes hard to enforce, as sellers often either don't know and minor work might not even be visible for a buyer to question. Yes, if you add on an addition or finish a basement without pulling permits, this is something that is more obvious. If a seller replaced a pipe in a wall, who is going to know?
Third, it depends on the property. You wouldn't want unpermitted work on a premium home in a premium neighborhood. But if you go shopping for a deal on a starter home in a rural area? Much of the inventory will have unpermitted work, there may be zero local enforcement, and the buyer will likely be planning on doing their own unpermitted work :)
Home improvement stores really wouldn't exist in the way that we know them if everything needed a permit.
> If a licensed contractor sees that you're trying to fix unlicensed work, they too will run away because they don't want to inherit the liability.
Many will be hesitant to fix shoddy work, but most will be fine with replacing it.