Another option is to use PAD. That stands for Portable Application Description. Read FAQ here http://www.asp-shareware.org/pad/padfaq.php
Once you got PAD that describes your software, you could use PAD submitter tools to submit it to a lot of shareware sites.
HTH
Off the top of my head, YC company BaseShield has a Windows app store. I'm not sure if they're open to app submissions, or handle purchases, though.
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.