I gave it a whirl and it's darned easy to use as implied by the 1-2-3-4 steps indicated on the main Store Builder™ page (shown in the screenshot to the left).
Just log in, choose the merchant or merchants that you want to promote, filter results by keyword, do a little color customization and grab the code.
Important note for WordPress bloggers – click the “WordPress Optimized” box located beside the “Select Code” button before you copy your code, otherwise the code won't work.
Here's an example of how I used the Pepperjam Store Builder™ to place BackCountryEdge products (filtered by the keyword ‘tents') on the Roamsters travel blog — using the default color settings. The actual page is located here.
With a little more tweaking, someone with good color sense could make that look MUCH nicer and more integrated with the page.
I don't know that building product feeds gets any easier than using the Pepperjam Store Builder™. If you do, please tell us about it by leaving a comment below!