I don’t really get it here. Who has such problems? People buying themes from 7Theme? Because I’ve never had such a problem before with other designer’s themes. It’s obvious why people want to use child themes – to write their own CSS classes or functions and don’t loose all the changes after theme update. So maybe your themes have some issue with that to prevent people from making child themes? Why don’t your themes work even with ‘clean’ child-themes, without any CSS classes or functions in function.php?