ecommerse, products help

I am new to WordPress and would like some help understanding ecommerce, catalogs, and products. I have inherited a site that badly needs updating – new theme and organization (original developer in not available for consulting). There are 150 products defined with what looks like a custom cart and checkout. When I install a catalog plugin none of the products or their categories are available and there is a new PRODUCTS field on the dashboard for product entry.

Do each catalog and/or ecommerce plugin have their own product and category system that is incompatible with other plugins?

Is there a way to export my existing products/categories and import them into a new ecommerce plugin?

Any information will help.

Thank You