@RXanadu
When thinking about getting games from 3rd party vendors who are selling Steam-keys, you need to cross-reference that by looking at the game's specific Steam-page.
You're basically looking for what is called STEAMPLAY support - click this link for details.
On a game's Steam-page, you want to Look for STEAMPLAY symbol and see what symbols for OS's are next to it - this means that the game-key activated gives you at least Windows + Mac versions.
Sometimes, this might ALSO includes Linux-versions, too - again, you need to check a game's page on Steam to see what's supported.
Examples:
In
Borderlands 2's case, it supports STEAMPLAY - when getting a Steam-key of that game from anywhere, you will get you both Windows + Mac versions.
In
Shadowrun Returns' case, it support STEAMPLAY - when getting a Steam-key of that, you'd wind-up with WIndows + Mac + Linux versions.