What do you think the lumen difference is using AAA batteries?
The lumen difference has more to do with the model LED being used. Using AAA will not lower output, but capacity is obviously lower, using 4-AAA will make up for that. If we knew what model LED was in the Costco we could easily say which is brighter. The lumen rating of a lot of the ebay flashlights are either on a totally different scale or are a theoretical max.