It's not possible to pick up the "sell" item if you use enchanted books with a StoredEnchantment in the "buy" slots...

summon villager -1100 70 -150 {Silent:1b,PersistenceRequired:1b,NoAI:1b,Profession:1,CareerLevel:1,Offers:{Recipes:[{maxUses:2147483647,uses:0,buy:{id:"minecraft:enchanted_book",tag:{StoredEnchantments:[

{id:"minecraft:mending",lvl:1}

]},Count:1b},buyB:{id:"minecraft:bow",Count:1b},sell:{id:"minecraft:bow",tag:{Enchantments:[

{id:"minecraft:mending",lvl:1}

]},Count:1b}}]}}

It works without a problem if you don't use StoredEnchantments in the buy slots...