When you trade several times with a villager or have it a long time generated, you can't trade or even kill him. When you log out and in again, this little issue is fixed.
PS: This is a bug, wich is since a long time in the game. Only now I had the idea to write an issue report. I've searched something like "Villager Bug", but I couldn't find this issue. I guess, this is maybe a duplicate. I am Sorry, if this is true.
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.