Is there a way to trade (not sale) games that I own on my library?
If there's a thread answering this please reply me the link. Did a little search but found nothing...

also if this is in the wrong place, please delete.

Thank you all.

Nope, there's currently no way of doing that on GOG.

One thing to keep in mind, is that you can only do that within 30 days. Once 30 days have passed, the sale is considered final.

Thank you ALL guys.
Exactly what I needed to know!
God bless.

You currently can't and since they're DRM-Free it would easily promote piracy, unless GOG puts some "Has Game Been Downloaded?" check. Might be doable in the future.

Since the beginning GOG had that flag in place and active. Back then, when logic and "customer love" still prevailed they converted a "game shelf" into a "gift code", as long as you never downloaded the game, no matter the time since buying.
I know that because I did that for a couple of games.

Now, any attempt in doing the same will get you a standard copy-paste reply "we can't do that because more than 30 days had passed since the initial buy".

There's a wishlist for this, hope more will vote on it:
http://www.gog.com/wishlist/site/lets_us_gift_away_games_from_our_library_that_we_havent_downloaded_yet

Guys, then how does everybody trades games in the forum? Where do they get the codes?
(I know I sound like a noob, but I guess I am then)

I can only speak for myself: I buy them cheap during sales, get them for free during sitewide promos, or win them in a giveaway and then turn around and regift them.

Others might have extra codes from sites such as Desura or Steam. Many times those places will provide extra keys, or bundle extra games that the OP (original purchaser) has no interest in playing.

Got it! Thanks!

Get them from Humble Bundles, from cheap bundles, Basically any time there's a good sale I try to pick up a gift code or two to donate to TBBBBOB. Some people just spend money on it. Some just get some benefits.

Since it wasn't specifically mentioned by TwoHandedSword, and if you haven't noticed it yet, when you make a purchase from GOG, you can tick a box to make the purchase a gift - enter your own e-mail address and you can save the code to give it to whom you want at a later time.

On the checkout screen during a purchase, you can choose "Gift this order" (if you already have the game in your library, that's the only option). Then you have to put in the recipient's email address, which in such a case would just be your own. After paying for your order, an email containing the gift code will be sent to the specified address and the code itself will appear in the "gifts" tab of your GOG account, where you can also check which games it contains and whether it has been redeemed yet.
You can try this out by simply "buying" one of the permanently free games as a gift order; for example: Ultima Worlds of Adventure 2: Martian Dreams.

@Maighstir
@InfraSuperman

That's something I didn't know. You're so thorough.
Now it's all clearer to me. The whole process... In my mind I thought I could just trade any random game in my account library once I played it, so I can get an other one to play.
Man, I need money...