believable trade? civ 5 + god and kings dlc for my dungeon defenders + DLC?

Believe it!

I see what you mean, I wouldn't try it. Could always add them and see if they'll go first though (assuming they're all keys).

It't doesn't sound very odd because the DDComplete is not in a bundle anymore, but check his steamrep.
Maybe ask him if he'll go first.

How will you sleep at night?

I traded my Bastion for Civ V and Gods & Kings not so long ago, so it certainly could be legit.

I would have him go first. How is his steam rep? Can you see his inventory?

You're either about to get scammed or sharking.

In case you don't know, sharking is a term used in TF2 trading where an experienced person takes advantage of a new person not knowing the values of their items. Pretty much the same concept here, you're aware of the value you paid and that Civ 5 has never been that low when the other person probably assumes you paid more than a couple dollars for your keys.

Maybe he bought the gold edition recently and just wants to offload his free extras? Still, I'd ask if you don't want to take advantage of someone.

Well, Civ5 Gold (Basegame + DLCs + Gods&Kings) is on sale for about 11,50$ right now. Concidering DD-Complete was a BTA-Game and is not available as such anymore, it would be a good deal for you, but neither unrealistic nor sharking.
Still, going first is always risky.

Ask to see his SteamTrades profile to see if he has a good amount of rep. If he doesn't, make him go first or get a middleman you can trust.

Check his rep and trust ur instinct.. I once trade Walking Dead Survival Instinct for X-Com, Max Payne 3 + Rockstar pass, Spec Ops (Tradable) and Bastion (Tradable)..

If ur unsure, ask the person to go first.. If key, once they go first, ur safe.. If gift, once activated, might get revoked, but just contact steam and they'll give u back..

So should be safe..

Well, if those Civilizations comes from Russia, they could have costed around $5 (today's civilization 5 and gods dlc price is 250 rub. each, so with -75% they would cost 125 rubies, which is less than $5).

And there's sale on muve now. It isn't that impossible.