There is more to the decision than simply "used" or "new".

Depending upon the amount of labor involved in removing and reinstalling the part will typically govern whether I consider used.

I don't normally use 18 year old parts that just happen to be working the day you buy them. How much longer can they be expected to perform? The price would have to reflect this. If I can't pay 25% of new, I probably won't buy the used part.

Of course, when speaking about trim parts and body parts, the whole equation turns to used, naturally.