In Descent second edition it says on most search cards that if you use them that they should get flipped over. In campaign mode does this mean that you still can sell them in the campaign phase?
Yes. That's why they're flipped face-down when used rather than being discarded--to keep track of how much gold you've earned. Conceptually, the card represents that your search uncovered a cache with both gold and a useful item.
From p. 10 of the rulebook:
Gold: When playing the campaign, this number shows how much gold the players have earned, in addition to the abilities listed on the card.