By itself, it is not a proper noun.
Any word or expression used in some special sense could be the proper name of something. Usually there is a context clue of some kind. In that case it would be capitalized. But we don’t go around capitalizing all the words in the language just because they might be the formal name of something somewhere. For starters, just looking at the names of a handful of Fortune 1000 corporations here in Silicon Valley, we’d have to capitalize these words every time we used them:
adobe
apple
intuit
juniper
oracle
sun
But we don’t. We capitalize them only when we are speaking about the entities that have adopted them as their names.
In the absence of other indicators—i.e., by itself—“fair trade” is not a proper noun. Lowercase.