Putting aside the fact that derailleurs are relatively cheap and buying a new bike to replace a seized derailleur is not economical (unless you get stuck on an upgrade path that makes you also replace the shifters). And putting aside the fact that bicycles needs regular maintenance and regular riding to stay in top condition (seized derailleur only happens if you basically leave it sitting in the rain, or never change gears)... The lemon issue only really applies to bikes that you buy sight unseen. You want to check for cracks in the frame and a worn drive train. Both of these are relatively easy to learn how to do.

Having said that, I wouldn't usually buy used either for a variety of reasons. If you are buying a quality bike, the damn component manufacturers keep improving the components in incompatible ways. So if you want to replace components with new parts (for example you wear out the drive train and you want to replace it), it can be difficult to find them. This puts you on an upgrade path, replacing everything so that it is all compatible again. However, bike manufacturers get steep discounts on components and it is almost always worth it to buy a brand new bike rather than upgrade all the parts of your old bike. Buying a fairly low-mileage used bike will only delay things for a while because eventually you will have to replace worn components.

Of course you can buy used components to replace your worn ones, but it is harder to asses the quality of the components unless they are attached to a bike. Also, there is a huge market for stolen goods and as a person who doesn't want their bike stolen, I also don't want to buy stolen goods. Used components are practically impossible to verify that they aren't stolen. Either someone is replacing the components because they are worn out (in which case you don't want them), or the components are stolen (in which case you don't want them) or the person is upgrading to new components before their old components wear out (very small number of people).

So basically, you are stuck with:

- Maintain your bike properly - Ride it a lot - Replace cheap worn compnents with new parts - Buy a new bike when you get stuck in an upgrade path

(of course you never throw away the old bike because then you would have one less bike and who wants that???... so you upgrade it with new parts too... sell the car to pay for it since you are riding so much you don't need it anyway)