getting emeralds from trading with no spending

when i was trading with villagers, i noticed that after a while they stopped accepting items from you, after still trying to do the trade i kept the emeralds and the villager gave me back my wheat.

i would call it more of an exploit because it gives you free emeralds

1. get a villager who is selling emeralds (preferrably farmer buying wheat)
2. get enough of the wanted material to get aprox 20 emeralds
3. trade as much as you can to villager before he starts refusing your stuff do not exit out of the screen or it will not let you make any more trades
4. if you have done it right so far, than you will notice the villager will give everything you traded back to you, also he till take the emeralds back
5. SHIFT + click the emeralds in the trade box and quickly pick up the emeralds. if everything was done correctly, the villager will give you your stuff back because he is not accepting any more of that trade, but there will be no emeralds in your invintory to take back (because they are in your hand)

this makes it very easy to get materials that are supposed to be hard to get saddles, eye of ender, bottle o' enchanting, diamond, chain mail, glowstone, enchantments, etc.