Provided he makes it plain to the buyer that the frame is damaged then he has no other moral obligation. If the buyer is a scammer who sells it on as undamaged that is a separate issue and nothing to do with the OP. I have sold a driveable car for spares or repair as it was knackered and would never have passed another MoT. I do not know what the buyer did with it, he could have ebayed it as a good runner for all I know, but that would have been nothing to do with me, morally or otherwise. I was honest and upfront about it's condition.
How can you make it out to anyone that the frame is damaged? The frame isn't damaged. It's destroyed. That crack has a high chance of failure being so close to the bottom bracket and the torque forces applied there, if you put the power in at some point It'd sheer the down tube off and then the top tube will immediately follow, literally splitting the bike in half.
It's one thing to sell your old car that'd break down, but its another thing to sell your old car that'd... well, break in half.
Carbon fibre is incredibly good at alerting you about when a crack has occurred, but it's not so good when that crack comes to fruition. It's a very toxic product to ignore when it shows the warning signs.
You can choose your moral obligations as you wish, I just don't think it's good practice to put up destroyed goods online and advertise them as 'damaged, possible repair' or anything alike that. It doesn't sit right with me.