I'm not an
SEO expert, but some things I noticed.
The biggest is that you are in an EXTREMELY competitive field, so you are going to have to optimize for niche terms, ie "Hewlett Packard Black Inkjet Cartridge #015" instead of "inkjet cartridges". You might try doing this for items that you have in stock that are hard to get elsewhere, for starters. That might help conversions.
Your code to content ratio is pretty awful. Moving the javascript to its own file, and doing other optimizations on the code will help a lot. If your shopping cart web app allows it, you might consider changing the layout to a standards conforming layout using css to define it. If it doesn't, then you can at least move a lot of the appearance stuff into the stylesheet that's currently in the html, then don't define that stuff when you're creating new html pages and let the stylesheet do its job.
105 backlinks isn't very many at all, in fact, that's about three week's worth on a site that's been up over a year. I recommend doing 20-30 submissions a week working through the
Vilesilencer SE friendly directory list for starters.
Articles about your field submitted to the article banks might help. I don't know if you have anything newsworthy to do Press Releases on. You can also consider adding a blog and blogging once a week. Pinging the rss ags when you blog will get you backlinks. And getting on some blogrolls will get you more.
I'm sure there will be others who chime in with more advice.