tag is responsible for telling the trade what tags the item has, BlockEntityTag for telling the item what tags the block has. So you need both:
summon villager ~ ~1 ~ {VillagerData:{profession:leatherworker,level:5,type:plains},CustomName:"\"Banner Clerk\"",Offers:{Recipes:[{buy:{id:golden_hoe,Count:1},buyB:{id:golden_axe,Count:1},sell:{id:red_banner,Count:1,tag:{BlockEntityTag:{Patterns:[{Pattern:"drs",Color:4},{Pattern:"ld",Color:14},{Pattern:"rud",Color:14},{Pattern:"mc",Color:14},{Pattern:"dls",Color:4},{Pattern:"ld",Color:14},{Pattern:"moj",Color:4},{Pattern:"tr",Color:4},{Pattern:"cbo",Color:14},{Pattern:"tts",Color:14}]}}},maxUses:9999999}]}}
Instead of using /data modify, use /data remove.
/data remove @e[type=villager,name="TheVillagerToActUpon",limit=1] Offers.Recipes[{buy:{id:"minecraft:stone"},sell:{id:"minecraft:stone"},Uses:5}]
Replace the 5 in Uses:5 to say how many times the trade can be used. Put the command in a repeating command block.
Best Answer
In 1.9, potions no longer rely on the damage value of the potion item to determine what type of default potion it is. Instead, the
Potionstring tag holds a string-based default potion ID, which is stored in thetagcompound:If instead you want to sell a potion with custom effects, you would use the old
CustomPotionEffectslist (using an effect ID):