I don't know about other people, I can speak only for myself. Even though I didn't sell any sites yet (I have have experience merely with client languages/SQL), I'm not satisfied with the current Javascript+HTML+CSS offering. I just feel it could all be done in a more concise way instead of dragging a thirty years old technology along the way. On my to-do list, is scheduled a replacement for those three riders of the apocalypse in a form of a self-sufficient single unifying language that compiles to those three. I'm thinking of something like
Elm, but more flexible.
But, don't take me for granted, my knowledge about broad site development isn't that much wide that I could recommend this or that tool from the plethora of currently available addons. Anyway, knowing Javascript+HTML+CSS+ServerScripting would give you a good measure of addon libraries you'd potentially buy and use. With that knowledge you could estimate how long would it take for you to create similar libraries and decide if it is worth of trouble. Probably there are some overpriced products, but I believe most of them reasonably save your valuable time when building new sites. You just have to find and pick the right combination for the right project.
And what that combination may be, maybe someone else could recommend something more meaningful. Informations that could help to answer your question are: what type of sites do you want to build, do you need server side scripting, databases, maybe some online store requirements, what is complexity scale of sites you want to build, amount of visitors you try to reach, how many time you are prepared to spend on your projects, ...
(For smaller and extremely simple sites [because of the library properties],
my own library comes with royalty-free restrictions for all CodeForum members, without any purchase.)