Without seeing what is being sold it is not possible to be sure what is being offered. It may be that what is being done is making what are called 'child themes'. A child theme is a modified version of a parent theme where the modifications are stored in a single, seperate folder.
There's quite few themes that are designed to work with child themes and making child themes is much faster and easier than making a theme from scratch.
If the vendor has purchased a developer licence for the theme then selling the child theme bundled alongside the original theme is perfectly OK, but the chances are that the theme can only be used on a single site so this willl be an important thing to check before spending any money.
There are some tools for making themes using drag and drop techniques but, having tried them, I have not found one that would make themes I'd want to sell (or use!).