Trade Problem...What Should I Do?

Leave a negative rep on his profile.

should've canceled the trade when he said he randomly found the link, or u had to make him send the link first
leave negative comment on his steamtrades, im not sure if steamrep.com covers such... scams

Fornicate him right on the genitalia

Better never trade for bundle keys unless person is selling em for cheap.

As an experienced trader I can tell you, the worst thing you could do is to outright call him out and leave a negative rep. Making an enemy out of potential sellers/buyers is not a good idea.

It's partially his fault for not knowing the contents of the key, and partially yours for not asking him to go first, neither is really to blame, but both made mistakes. Yes, the whole "found" thing might seem a bit shady, but chances are he found it in his email inbox or in a text file on his desktop from a trade he made a while ago/ when he bought the bundle. So give him the benefit of the doubt unless you're seeing red flags in every other aspect of his behaviour.

The best thing you could do right now is to explain the situation, ask for a bit of compensation (like a few cards back) and then move on, no point dwelling on something like this. You'll be able to buy the remaining two games eventually. Take care and trade smart. :)