Assassin's Creed Black Flag, Price for Steamtrades

Offer? As low as possible. Probably not higher than historical lowest sale price.

Trading for money, not a good idea.

Easiest is to just check Steamtrades. It's not uncommon for people to say what they are willing to offer in the thread title, so you can just skim those to get a good idea. Note that the currency that's most often being used is keys. If you want "real" money you'll get slightly less than the value of the keys. Also, if you don't have a good rep score, people might not want to trade real money with you, as the risk of scamming is greater.

Click me for list of offers

I would say 5 TF2 keys or $10-13, but only for UPlay, since it was free with Nvidia cards.

Probably $18-25 or 9-10 keys for Steam version.