Many people do not want to play in mega guilds. They want to just play with friends and family. Also, even a market of 500 in a mega guild is too small for an efficient economic system so even if many small friends and family guilds come together in a separate mega guild for guild store purposes, buying and selling, and thus crafting, will be difficult and time consuming.
Instead of forcing these mega guilds on everyone, let guilds come together in consortia for the purpose of buying and selling only. We could then be in large or small guilds as we see fit and not have to be in multiple guilds dealing with the issues of managing membership in each, and we could form markets much larger than 500 for a more efficient economic system.
Also, please update and improve the guild store interface ASAP. It is horrible as is. Something as simple as a keyword search, in the buying tab, would be a tremendous improvement.