Child themes require the use of the parent theme to be installed in order to work (they don't work on their own), so this is not the same as if they were selling our actual themes or other products created by us, and hence is not an issue.
Some marketplace developers do in fact have other child themes/plugins they sell which they themselves choose not to sell in the AppThemes marketplace. Products that are offered by developers to sell on our marketplace are required to sell those exclusively through the marketplace, which is why they may decide to select only certain ones. There is a certain amount of vetting and testing required before products are accepted into the AppThemes marketplace, so not all applications are accepted. Thanks.
