Buying at third-party key sites is always at your own risk.

引用自 Ettanin：

Buying at third-party key sites is always at your own risk.

I am aware of that. But I have also received and redeemed such random keys here on Steam. What interests me is whether you can also get games that you already have.

While such sites could ask you to provide your profile information so they can check which games you have, it's unlikely that they actually do this. Therefore, you can get keys that you already have.

And sure, you can try to sell them. It's not going to work, though. There is no way to safely trade keys which makes this an extremely high risk transaction for whichever side "goes first", and nobody will be buying rubbish games like that anyway.

引用自 Kargor：

While such sites could ask you to provide your profile information so they can check which games you have, it's unlikely that they actually do this. Therefore, you can get keys that you already have.

And sure, you can try to sell them. It's not going to work, though. There is no way to safely trade keys which makes this an extremely high risk transaction for whichever side "goes first", and nobody will be buying rubbish games like that anyway.

But I also received keys like that as a reward on Steam. I thought that's what they were talking about. However, I've never bought external keys because you always get a better price in the sale in the end!

引用自 SirRantsAlot：

But I also received keys like that as a reward on Steam.

No, you did not. Steam does not give out "rewards" for anything, and they have never done "random keys" in any context (at least not in the last decade or so).

In fact, Steam does not hand out keys to users at all; only publishers can request keys for their games.

引用自 Kargor：

引用自 SirRantsAlot：

But I also received keys like that as a reward on Steam.

No, you did not. Steam does not give out "rewards" for anything, and they have never done "random keys" in any context (at least not in the last decade or so).

In fact, Steam does not hand out keys to users at all; only publishers can request keys for their games.

I collected badges or similar items here (dont remember cause it was years ago) and exchanged them internally at Steam for keys, and then won a few junk games. I certainly didn't buy them anywhere else.