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.