If you talk to the tube guys who really know their stuff, they will tell you that tons of tube testers out there are inaccurate, uncalibrated or wrongly-calibrated, and the honesty and expertise of many tube sellers is questionable.
This nonsense about used tubes measuring as new needs clarification. Besides the factors listed above, it is true that tubes can test fairly high for a good part of their life, then suddenly start to rapidly decline. Lesson? It is foolhardy to trust many tube resellers who proclaim themselves experts and make claims on how close to new their tubes are. Caveat emptor applies here. Unfortunately, you have many foolish purchasers who think they have gotten what they paid for, leave positive feedback, and then the prospective buyer believes he is dealing with a reputable seller.