Because of the enormous amount of visitors and marketplace members Themeforest.net is at the moment the best sales channel for us. It reduces the need for expensive advertisement and helps us to keep our product prices down.

You can of course use our WordPress E-Commerce themes for offering a different product catalog. All you need to do is to exchange the product pictures and adjust the shop title - which you would do anyway. "Zionroasters.com" e.g. uses our "FurnitureStore"-theme to offer coffee.

There is no limit of how many products you can list. The shop with the highest number of products we know of has currently about 925 items in the catalog; with no signs of performance loss.
Our Wordpress E-Commerce packages use the native post structure for products. So basically 1 product <=> 1 post.  Means: the maximum number of products is equal to the maximum number of posts WordPress could handle.