I purchased some games from someone else and I have the original discs, cases and product codes. I cannot activate the games because they were activated by the previous owner, whoever that is. What are my options?
There is no way for you to legally play the games you have bought if the activation codes have already been redeemed. Your only option is to take them back to the person you purchased them from and ask for a refund.
The games and their activation codes are locked to the original owners Steam account and cannot be transferred to your account. The discs you bought are effectively worthless as although they contain the digital files for the game they no longer include a license to play the game, this was redeemed by the original owner and they still have the license and are able to play these games even though you now own the physical discs.
PC games almost always have one time activation codes, once they have been activated, in this case by the previous owner, there is no way for you to activate them again on your, or any other account. To play these games you need a new activation code, this is why there is no market for second hand PC games at your local game store or anywhere else.
Assuming this was a private sale, I don't know how viable an option returning them really is. They probably sold them to you knowing full well they were not playable and you will be relying on them agreeing to give you a refund.
If you bought them from a store then you will be in a better position to return them, depending of the laws of your area.
You should ask for refund!
I purchased the games (about 10 of them) from the ShopGoodWill.com auction website. I did basically what you said and reminded them that they sold me previously licensed software that is basically useless/worthless. That is all it took. They issued me a full refund.