A) Most likely that you don’t have Mod_rewrite as part of your server package.
B) Trial maybe causing errors due to it’s encoding
C) I’ld suggest you take advantage of cs-cart’s 30 money back guarantee just in case it’s not right for you.
for SEO-urls you can keep them, just needs to be added at the bottom of everypage you create. otherwise it’s an automatic process.
I also went from paypal buy now to CS-Cart
Since my products don’t change much, one thing I did was keep my original home page as a static html page (highly ranked, fast loading, small, css designed) and then from there linked to cs-cart pages. It has worked out great.