Actually, I'd suspect that a 70% bounce rate is about average for your niche but implementing the changes I've recommended should help, at least to some degree.
With 400 unique visits a day, if they're spending an average of 1-2 minutes browsing your site, you've got ample opportunity to set a hook. Fix the cart issues and you'll see some improvement. Improve the overall shopping experience and you'll see some improvement.
Check out some competing sites to see how they're tackling the same issues. You'll be miles ahead if you do.
BTW, you'd do well to pick up a copy of Steve Krug's "Don't Make Me Think" volume 2.
Advanced Common Sense Home
He does a great job of showing folks how to dissect a website to diagnose and prevent problems like these. It'll help you see things like a user would, particularly where Navigation, checkout pages, page layout and copy writing are concerned.
It's an easy read, an invaluable resource and money well spent.
Don't forget to update the thread when you've made some changes.
Good Luck!