Is the eCommerce site Shopify SEO friendly?

It depends on how you define "SEO friendly". Have they taken some steps to improve SEO? Yes. Would I ever build a business site on that platform? No.

The nice benefits are you can get your site up and going quickly and they offer excellent hosting.

Aside from the financial expense of needing to pay monthly for the service, there are far too many limitations from any hosted solution. There are tons of SEO optimizations which are desirable but cannot be made due to you having no control over the software nor the hosting environment. Sure you can change title tags and some other changes, but that is not close to enough.

I would recommend WordPress + the eCommerce plugin or Joomla + VirtueMart over Shopify. I mention these two specifically because they offer a nice balance between a cart and a site. If you only need a cart, there are several cart solutions available.

If you do go with Shopify, be sure to get your SEOmoz discount: http://www.seomoz.org/pro-perks

Whats with the backslash in the url adding as duplicate content?

A backslash is viewed as a character. You could choose to present different content on a URL with a backslash versus without one.

mysite.com/testing does not necessarily offer the same content as mysite.com/testing/. Just because you choose to offer the same content on both pages does not mean search engines will understand your choice by default. Each URL is unique and viewed separately.

Is this a bug or something that needs to be addressed? If so, just use a redirect?

If you used the SEOmoz crawler and it notified you of an issue with some pages on your site which are viewed as duplicate content due to the addition of a backlash, it is not a bug. This issue should be addressed. A 301 redirect would likely be the best solution.

Mike, the one point to clarify is whether you feel that Thomasnet.com is performing well? or whether the sites which pay Thomasnet.com for links are performing well?

If Thomasnet.com is not paying for any links themselves, then there is no reason for them not to perform well.

My understanding is that Google would penalize the BUYER of any links by removing the value of any paid links. There is much speculation about whether Google takes any additional steps. They could penalize the seller and they could further penalize the buyer, but I am not aware of any confirmation to either point.

None of these sites are linked at all, and all 28 of the sites are under the same hosting account (all are subdomains of root domain of hosting account).  Question, should I link these sites together at all and if so how?

What you are asking about is creating a link network which violates search engine guidelines, and therefore is not advised.

A link is supposed to be an "independent vote" and unbiased in nature. You are simply taking a bunch of sites you own and linking them together. If you sincerely feel it is a benefit to link the sites, use the "nofollow" attribute to avoid a penalty.