I don't believe that anyone has suggested that lengthening chambers increases the value of guns (though it may if the gun is reproofed and no doubt has when undetected). The only question is whether, assuming that two guns are identical in all respects, maker, age, condition, gauge etc., except that one is a 2-1/2 inch gun and the other is 2-3/4, will they sell as easily and for the same price; i.e., be as marketable? If you were in the market for, say, a 1935 Purdey game gun, rose and scroll engraving, in original, not "messed with" condition, and were willing to pay tens of thousands of dollars for it, would you rather have it with 2-1/2" or 2-3/4" chambers?