I think I can get you at least part of the way there.
I created two calculated fields:
1. Duplicate Seller
COUNT([Seller]) - COUNTD([Seller])
2. Non-Duplicate Sellers
COUNT([BUYER]) - [Duplicate Seller]
I then selected "Pie" from the Marks card and placed one of the calculated fields we created in "Color" and then Buyer in "Details"
I hope this helps!
I'm not really clear on what you are expecting, looking at your data I see 75% of your buyers buying from the same seller, how do you end up with 50%? You might be able to solve this easily with an LOD expression. Can you explain what you expect buyer 7777 to fall under, it looks like same seller to me?