Query

• self-lookup and match if not same id (avoid same item on pair)
• map to add to the join result the parent item (the pair will always have as first member the one with the biggest price (needed for later))
• unwind pairs
• group pairs to remove the duplicates (each pair will appear 2 times, but its sorted from the map so we can group by it)
• find the price difference, sort by descending, unset the field

*i am not sure it does all you need, because i didn't understand the part hide items with the highest buy price the highest buy price is "2" and in your results you have this items

Playmongo

coll.aggregate(
[{"$lookup": 
   {"from": "coll",
    "pipeline": 
     [{"$match": {"$expr": {"$ne": ["$$id", "$_id"]}}},
       {"$project": {"_id": 0, "price": 1, "type": 1}}],
    "as": "pairs",
    "let": {"id": "$_id"}}},
 {"$project": 
   {"_id": 0,
    "pairs": 
     {"$map": 
       {"input": "$pairs",
        "in": 
         {"$cond": 
           [{"$gt": ["$$this.price", "$price"]},
             [{"price": "$price", "type": "$type"}, "$$this"],
             ["$$this", {"price": "$price", "type": "$type"}]]}}}}},
 {"$unwind": "$pairs"}, {"$group": {"_id": "$pairs"}},
 {"$project": {"_id": 0, "pair": "$_id"}},
 {"$set": 
   {"priceDifference": 
     {"$abs": 
       {"$subtract": 
         [{"$toDouble": 
             {"$getField": 
               {"field": "price", "input": {"$arrayElemAt": ["$pairs", 0]}}}},
           {"$toDouble": 
             {"$getField": 
               {"field": "price",
                "input": {"$arrayElemAt": ["$pairs", 1]}}}}]}}}},
 {"$sort": {"priceDifference": -1}},
 {"$unset": ["priceDifference"]}])