In Descent SE can you sell search cards after use?
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.