I have 2 old, unredeemed keys: Mount & Blade and Neverwinter Nights 1 Diamond edition.
I'd like to exchange them for other games, but first: does GOG allow that? and second: is there a safe way to do it without simply sending the key and risking not getting a key in exchange?

1. Look here https://www.gog.com/forum/general/gogs_general_trading_thread

2. Read the rules!

3. There is always a minimal risk. Better know your trading partner.

First answer: Yes, GOG allows it as long as you trade and don't sell the keys for money.
Second answer: No, there's no safe way.

My hints: Just trade with people you know and if an offer is too good to be true, it's mostly not true.;)

Didn't the mount and blade key expire already? what does it say when you want to redeem it?
Since we did get a new key at the same time and only had like 2 weeks for someone to redeem the key i simply gave it away.
I bought mount and blade in 2015

I haven't tried to redeem it. I have never redeemed keys on GOG, is it a one-step function?

You pop in the key, it informs you of who purchased it and the date of expiry and you are given the option whether to "use" it or not.
Captcha is involved in part of the authentication.

Scrool down to the bottom and click the redeem code, then if you have a code for mount and blade enter the code there and then continue. You can cancel the redeeming if you don't want the game to be in your library if you already have it then you can't redeem it but only gift it to someone, check this way if the code is still working.
I kinda doubt it works since at that time the code expired without giving us a proper date of expiring and gog offered us a different code so people could redeem it which had an expiration date of 2 weeks i believe.
Oh maybe you will be bothered by captcha, maybe not. Hasn't bothered me for some time now.

Indeed, the Mount & Blade key does not work. It says "Oops, something went wrong. Please this page and try again. If you're here about code MMLXXVII, please be aware that it is invalid and not part of an official promotion." By the way, the security captcha system is very, very anti-human instead of anti-robot!
The Neverwinter Nights 1 Diamond key works fine, thankfully. Thank you all for your help, I will use the Trading thread to exchange the one key that works.

Maybe you should give the keys away. if you do not need them it would be fine.
Do what you want to do.