I believe I found a logic error in this addon.
The table: product_sales has 3 fields:
category_id product_id
amount
I have several Products which are in several Categories:
category_id product_id amount 
53 470 244 125 470 244 132 470
244
The Bestsellers add-on adds the 'amount' field together so the above Product #470 is calculated as being sold 732 times when in fact it has only sold 244 times.
Whereas, Product #865 has been sold 639 times:
category_id product_id amount 158 865 639
The way Bestsellers is coded, Product #470 appears to be outselling #865.
Running Multi-vendor V3.
Nevermind … my mistake.
“My issue” with the Bestseller add-on is due to the way I was migrating the data from another shopping cart.