Amazon sometimes sells Steam keys that have been used before.

Weird, I've never had issues with amazon buying keys. I did buy the bioshock pack from them too.

Have you clicked on the "click to view" button for those keys? If you're not going to use the key in a while, perhaps not clicking that button until you intend on using the key can help prove that you have not activated the key yet. (Not sure if Amazon actually keeps track of that kind of info)

Maybe try getting in touch with Tony? He's the Amazon digital downloads guy that does all the community outreach, you can find him on several big forums and/or reddit. Super nice guy, explain what happened and he might be able to help you out, or at least inform you of what happened.

"too old to have replaced because they think I played it and just want my money back" If you just ask to replace the key, how is it wanting money back?

I never personally had that kind of problem but traded with few people who gave me duplicated Amazon keys (like Payday ones). I've not been scammed, they always gave me working keys after but they were SURE that they never used the previous ones.

It happens, I guess.

A friend of mine got a used key from Gamersgate once.

Bought hundreds of keys on Amazon and only had 1 duplicate key for a Sonic game and was told to contact Sega. ._.

so far all keys from amazon worked for me , realy great shop

I have 80 steam cd-keys bought in amazon and I haven't encountered at least 1 invalid/duplicate cd-key.
prolly a rare-issue or a cd-key that has not been shown (the show cd-key) for a long time