If your variants exist as single product you will find several products beginning with the same name but ending with the variant name e.g. the color. 

To export products to your marketplace we will need to group those variants as you only want to display one product and not all variants of the product in the marketplace. 

 

In this example one product "T-Shirt NOLDI" exists in 3 Variants differentiated by color as single product "T-Shirt NOLDI OldRose" etc. 

We now will habe to create just one product "T-Shirt NOLDI". 

Therefore, select all variants, click on Group on the upper right and type the name of the product without the variant (in this case color) in the pop up menu. 

The successfully grouped product variants will display the parent product on the right.