I tried to summon a custom villager.

Expected: Snowy farmer,  that buys 14 wheat for 1 diamond.

Actual: Snowy farmer for a split of second, then a villager that has no trades.

Run

/summon minecraft:villager ~ ~ ~ {Offers:{Recipes:[{buy:{id:"minecraft:wheat",Count:14b},sell:{id:"minecraft:diamond",Count:1b}}]},VillagerData:{profession:farmer,type:snow}}

and you will get just a snowy villager that was a farmer for a split of a second