Free. No money ever changes hands.
Steam handles the sign-in — KeySwap never sees your password. Codes are encrypted before they are stored, and only revealed to your swap partner after you both accept.
By signing in you agree to the terms. The privacy notice says what is stored and how to delete it.
Bundles, gifts and giveaways leave most of us with spare Steam and GOG keys for games we already own or will never play. On KeySwap you list them, say which games you want instead, and swap a key for a key with somebody who has the opposite problem. It is free, and no money ever changes hands.