Does Steam Trading mean I can sell my used games? No. Only games that have been granted as an extra copy, and thus have never been played, can be traded. Once a Steam Gift is opened and added to your game library, you won't be able to trade it again.

If you have not had Steam Guard enabled for 15 days, you will be unable to trade or use the Community Market. Accounts that currently have Steam Guard disabled will be unable to trade and use the Community Market.

Total game size? According to the store page, Black Ops III requires 60 Gb of disk space. The store page of the season pass also mentions a requirement of 60 Gb of disk space.

Storage: 60 GB available space.