On the singleplayer campaign, I tried to sell some old rusty swords I didn't plan to use. There was no option. Didn't see anything in the manual about selling either. Am I correct in thinking the engine doesn't support it, or just not the default campaign?
Selling items is currently not implemented into the game. As far as I know: doing so is possible but would be very very weird and difficult to implement unless Craig added in new actions that would make it easier. I wouldn't expect to see it any time in the near future.