Excellira - 6:24 am on Feb 8, 2010 (gmt 0)
Merchants are afraid because their data is under someone else control
This is a positioning issue. While I'm not extolling the virtues of third-party or self-hosted data, if you are targeting very small business you may consider taking the position that you are solving a real and serious security issue (PCI Compliance). By providing a hosted solution you are able to manage and tighten data security better than a non-expert small business with limited resources and IT savvy.
Search traffic volume is low
Though it doesn't sound like you are, but when carving out a new category you have to strike a balance between dominating the new, unknown category and pulling traffic from traditional key phrases. The cart segment is substantial so capturing an audience is possible.