By changing the NBT data of a villager, one can modify a villager's trades. However, when I made a trade as shown here in part of the villager's data:

{maxUses:10000,buy:{id:emerald,Count:1},sell:{id:wheat_seeds,Count:8,tag:{CanPlaceOn:[farmland]}}

In the villager's GUI the wheat seeds displayed in the left slot, but there was nothing on the right and when wheat seeds were put in the trading slot for the villager the game crashed.

At the time of this crash there were about 11 other villagers in the world with modified data and a standard hopper clock running to operate multiple command blocks.

Attached is the full code for the villager that was used.
(There are indentations in this to help with bracket balancing and nested strings)