I’ve owned twelve Leica 50mm Summilux ASPH lenses in my lifetime, and three of them have been perfect in build quality and astoundingly sharp at f/1.4. The rest have been just short of exceptional. Most people wouldn’t notice… because they’ve never been crazy enough to buy, use, and sell multiple examples of them.
And don’t get me started on how many new Leica lenses will arrive with loose aperture rings, or wobbly built-in hoods, or back-focusing, or front-focusing, or… etc.
The moral of the story: in the case of Leica lenses, it’s actually better to buy used vs. new in a sealed box. You not only save on price, you also have the opportunity to examine and photograph with the lens you’re interested in before purchasing.