Complete safely is out. Someone would need to go first when it comes to keys. Given your 0 / 0 rating on SteamTrades, it would almost certainly be you. You will just need to find a trustworthy member to trade with there to keep things as safe as possible. Only trade with people you added directly from the site, not random friend requests.

There's no "safe" way to trade anything outside of the steam trade window. Paypal money can disappear, someone could simply take the key and laugh at you, etc.

They don't know your key is good/works in their region/whatever, and you don't know if they will actually pay for it/if the payment will disappear next month/if they lied about the key not working/etc. You can try to reduce the risk but there's no way to eliminate it. Good luck! :)

Please keep in mind that FC Blood Dragon keys for UPlay might be region restricted.

There are ROW, EU and US keys

Edit: seems there's currently a sale for the EU uncut version for 1€

Wrap it up in a wet towel and don't trade unless the person on the other end goes first

Complete safety? Not going to happen.
Your best bet would probably be to use a well-known middleman

What do you want for it? :D

Find people with a large reputation and a very active steam profile with many games and multiple years on steam.

You should meet with other party in a crowded place, engage in a Mexican stand-off and simultaneously activate codes on a laptops (which is kinda tricky, because you will need one hand to maintain Mexican standoff). Also remember to check the place for snipers on arrival.

Also check if this code isn't region locked. Some steam games from CD-Action are Region Locked to Europe or Poland.

And note that those codes are for you or your friends. It is illegal to sell them. At least in theory.