in my 30 years in photography I have never heard such nonsense.. just because a lense has been out of a box does not make it unsharp, companies like canon,leica etc have very good qc..a lemon is rare unopened or not
No, the original poster has some good points; it's not nonsense. Being in an opened box does not suddenly make it unsharp of course, but a lens that has been returned has been returned for a reason, and there's a reasonable chance that reason is because someone has tried multiple lenses and returned the worse ones. The original poster doesn't want to end up with someone else's reject.
Also, I'd dispute that lemons are rare, just on the basis that the second lens I ever bought (and I've only bought five total) was one. I got a "lemon" Canon, a particular lens that other people regard relatively highly for a non-L lens, but mine was quite obviously blurry in the corners compared with other much cheaper (non-Canon) lenses I've used since. Maybe I'm a statistical anomaly, but I doubt it.
The original poster has some valid concerns. Unfortunately, I am no help in answering them.