Definitly quite interesting , would probably be interested if the site is well made and reliable

Hmm, that sounds good for key traders.

If Steam found out, they would do everything they could to close it :S

Steam don't want keys being exchanged/traded. Only activated.

IMO sounds like a neat idea for trading keys, though for me i wouldn't need it. I trade in keys and trade-ables and if you're careful you can avoid a scammer even if you go first with keys, BUT i could still use it as a pre-caution as well.

I have a +45/0 Rep on steam trades alone, a good chunk of which were keys.

How do you plan to monetize?

"Both can enter their keys"

yeah ... no.

Key trading is already risky enough, I would not want to enter my keys somewhere else.
And how would this prevent scamming anyway? (I mean the method of the key exchange. checking rep sounds good).

You can already rate and comment on a trader on steamtrades.com, which allows a wider user base, because it's not restricted to key trades.

The biggest question is.. How can we trust you?
I mean, as it has been pointed out, trading keys is risky. Now, instead of "only" losing the key to a potential scammer, I can lose it to you as well.
And then there's all the other possible middleman things you could do, all of which result in me losing my key.
Even if you start out without, say, logging keys etc., who's to say you won't do it further down the road?

I would only support a project such as this if it were open source.
Edit: And even then, I'd be wary without having hands-on experience with it.

Sounds like a great idea, but maybe change the name to "man in the middle." :)

Run a legitimate service for a while, then claim "Oops, we was hacked!" and steal everyone's keys. Or have it checking your wishlist, and when someone tries to trade one on your list, you quickly redeem it on your own account.

Seriously, instead of having to trust one person, now I would have to trust two. Good luck. :)

That's a good idea sir!

If Valve added something in the Steam client, like "Activate a Product on Steam..." to create a gift from a key. That is a service I would trust. :)

Would be a great way to scam people.

So what if one trader trades a fake or already used key?

As has been noted by others, I believe this is a useless idea. It only serves to create a false sense of security for people that lack the common sense to trade without such crutches.

If I was a scammer, I could still use your site by creating alts with clean rep profiles and then trading invalid keys to people that think they're safe because they're using a middle-man. If someone accused me of scamming I could claim my key was valid but that the other party had simply not activated it on his main account, or that even you or a hacker had intercepted and used the code before the other party got it. In any case, I would still have the other party's key because the swap would be simultaneous.

I have 124 confirmed good trades on Steam Trades, not to mention countless other trades involving a) those same people, thus no duplicate rep, b) people that don't use Steam Trades, c) TF2 goods. I deal only with reputable people and/or those that seem trustworthy (believe it or not, most con-men can be seen coming a mile away). I never go for deals that seem "too good to be true" partly because of the odds of being scams, and partly because I do not seek to shark people. I have gone first plenty of times, and I have had others go first. I never have used a middle man, and I never have been scammed.