Realistically, the quality of a used Benge 3X is going to be largely based upon its serial number (year of manufacture). In other words, is it a Chicago Benge (early '50s and earlier), a Burbank Benge (early '50s to 1971), a '70s LA Benge after King bought the company, or is it a late '70s or later Benge in name only, manufactured after UMI moved operations from LA to Elkhart?
There are some who firmly believe Benges with serial numbers above 8XXX - 10XXX are inferior. My personal opinion is not that restrictive. I bought my LA Benge 3X (serial number 12XXX) new in 1974, still play it, and it has been a great horn in both build quality and tone!
For more Benge info, go to
http://www.dallasmusic.org/gearhead/Benge.html