I'm not sure how much you can sell it for but you certainly can sell it for any price you want. I wouldn't sell it for cheap but I would give it away at a decent price that will make yourself and the person buying it happy.
Are you switching to another package on whmcs or a total different billing system.
You can sell it, however I believe they have a $20 fee to transfer ownership unless it has changed, you can cover that or the buyer, maybe sell it 10-30% cheaper than what it cost, that gives people a reason to buy it from you
Yes, it would have value. Where did you purchase this license? If it was directly at WHMCS (and not from another person selling WHMCS) you can sell the license at any price point you feel is reasonable + the $20 transfer fee (You or seller covers this).
Make sure to include when the updates/support expires in the ad thread.