In the buynow module settings under the product listing setup section you will see the following radio button options:

• Hide Search and Sort – Check this box if you would like to hide the search and sort bar at the top of the product listing page.
• Show Search Bar – Check this box if you would like to show the product search bar at the top of the product listing page. The search bar does a wildcard search on the following fields:
  • Product Sku
  • Product Name
  • Product Summary
  • Product Description
• Show Sort Bar – Check this box if you would like to show the product sort bar at the top of the product listing page.

To display the search bar at the top of the buynow module you need to select the second option "Show Search Bar" and the search should work well for you.

-Scott