As said before, you can sell them to vendors for 50% of the actual value. I believe if you sell them through your replicator (under the recycle option) it is 40%.

Honestly, I sell every item that I don't need through the replicator and I still have more than enough energy credits. The thing about this game is that there are many different currencies so energy credits aren't the most important and you don't need a whole lot of them.

Anyway back on topic, if you want to get the most money for your items sell them to vendors. You can also sell items through the exchange, but not all items will sell.

All starbases (and some planets) have vendors. Here is an interactive map of earth space dock. You can use it to see where all the vendors are. On that page there is also links to the maps of other stations. No matter where you go you will only get 50%.