Steam key buying help

Where should I check for cheap Steam keys before buying something? My son wants F1 2018 and I see that it can be bought cheaper on other sites but I don't know if any are dodgy.

cdkeys.com is the go-to site, they’re reliable and as far as I’m aware completely legit. I’ve used them plenty of times.

Sadly, the problem is less the site you choose (though that can also be bad) and more the source of the keys. cdkeys.com is good and legit, from what I hear, so go for that like Gar says. But you can never really tell where a key has come from, so there's always an element of risk somewhere down the line.

Thanks Mart, that's pretty much what I though the situation was.

And cdkeys was the site I was trying to remember so thanks to Gar too.