I don't have a lot of experience in selling software online, but as a consumer, these are the places that I'd be looking to find niche software.

1. Your own website (with Google AdWords campaign)

2. CNET Downloads (if you have a free trial version)

3. snapfiles.com (if you have a free trial version)

4. eBay store

5. Amazon store

Really, it seems that you'd want to push ALL traffic to your own domain where you can completely control prices, offers, wording, etc.

EDIT: I just checked out your site [http://www.watermelonexpress.com] and it seems like you've already done a lot of the leg work. The problem that I see is that this market (test prep software) is saturated. I think an aggressive AdWords campaign would be your best bet.