I just bought the Humble Origin Bundle, I already own one of the games in it and I used the steam keys for the ones that I could. Would it be legal to sell the unused Origin keys?

Under the table?

Selling Origin keys you own is against the Origin Terms of Service.

From the ToS:

4. Use and Expiration of Content and Entitlements/General License Restrictions

...In addition, unless expressly authorized by EA, you may not distribute, publicly perform or display, lease, sell, transmit, transfer, publish, edit, copy, create derivative works from, rent, sub-license, distribute, decompile, disassemble, reverse engineer or otherwise make unauthorized use of Content or Entitlements. Any commercial use is prohibited.

Ah so trading is ok

No, it's not.

My general policy with any Humble Bundle is to just ignore the spare keys. Just doesn't seem very ethical to sell them.

Just give it to someone. The trade market value of the game is going to be next to nothing now anyway with the game being on the humble bundle. Be kind, just give it away. From what I understand, just giving it away is alright.