I am very upset with how Bethesda handled this. If you sell your armor then their is no way to get the achievement or the guild master armor. Bethesda didn't think that people would sell the armor because they have something better. I play on console so I cant get it back to use Tonalia as a fence or get the master amour something needs to be done about this issue.

moral of the story: be careful what you sell

Bethesda also figured that if the people really wanted an achievement they could always reload a previous save or make a new character.