Enable trainers to buy back manuals or add error checking
Or
Give all merchants a 1 minute resale. Buy the item, oh darn I didn’t mean to do that. You have 1 minute to sell it back at full price, if after, things are as they are now.
Aurora Glade [EU]
I agree that it could be handled better. Actually, I don’t get the point of the manuals at all. They could have just done that in the traits UI(the same way you can buy additional bag slots in your inventory UI), not in the form of a vendor item. Even if they really wanted you to visit a town to be able to do so, they still could have made it a conversation option, just like learning/un-learning crafting professions.
Also it might be convenient to be able to retrait wherever you are, at a little extra cost or something. Just a thought.