Hello Nikita,
You can try one of the below 2:
1. Create a customer exit variable for Product. In its code, you will have a Select statement where you get values from the variable that you've used in your BEx query for Seller
The customer variablill then be populated with values for the Seller that the user enters
2. Try using a Replacement variable for Product. The replacement should be from a query which gives all products for a particluar seller.
Havent tried this option myself fully. Let me know incase you try any of these.
Regards,
Srilakshmi B