I recently purchased a used S&W 66 revolver in 38 special.
Uhhhh? A S&W 66 is the stainless steel version of the S&W M-19 (blue). Unless I have lost my mind (a possibility), they were chambered in .357 Magnum, not .38 Special.
Other than that, if you look through the offending chamber at a strong light, you should be able to see any evidence of fouling that would cause the case sticking problem. If the chamber is clean, it is not caused by fouling. In that event, put a straight edge along the cylinder over the offending chamber and observe if the chamber has been bulged by some over-zealous hand loader. If the chamber is bulged, you have a serious problem and will have to consult with the factory, or someone who can make/supply and fit a new cylinder. A bulged chamber may be the reason you found it in the "used" counter.