I think most of the players found themselves in a dilema between selling or exchanging the chests/gems/fragmets.
My opinion on that matter is divided by the options you have:
1. If you are a busy PKer / LVLer or just don't have much time to spend on your computer and don't want / can't let it running while you're afk its just the best thing to sell them to buying merchants, you get instant cash, well maybe its a bit less then you would get for selling them yourself or exchanging them but if you don't have time to sell its a great thing to do.
2. If you can let your PC running over night or have another PC to put on merchant its best to exchange the chests/gems/fragmets but not in a matter where you exchange them the moment you get them but accumulate a lot of them and exchange them all at once which will increase the chance of getting better items and then putting the items you collected on merchant over night or on the other PC
If you have your own opinions or want to correct me please reply on this topic.