The seller's describing something afoul with the ribs, either loose, or cracked solder that's allowed water between the barrel and rust to start.
When a good set of barrels are properly struck (rung), they will "ring" to the ear, indicating tight joints throughout.
Double gun barrels are thus tested via removing them from the action and suspending them from their hinge pin pivot notch while the side of each barrel is rung with a finger flick/knuckle or a small rubber hammer (in a shop).
In any event, a rib resoldering job is indicated at the minumum, or even a full re-ribbing job - a MUCH more expensive proposition, for non-DIY persons.
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