Based on the information available:
The OP is a business and his goal on eBay is to buy low so that he can sell to someone else high (potentially adding no value in the process).
He makes an insulting offer (based on an end user expected resale price). The person receiving the bid looks at the profile and their transaction history, sees they are a business, knows the offer is low ball with the goal to resale higher, and expletively calls them out. They likely see little loss in doing this since the OP is not their target customer and is unlikely to be one again.
If you are buying $5000/week, you know what a realistic end user price is and likely offered less, perhaps far less than that. You know it. Likely that seller knew it too.