I will try to answer a few of your questions...
Kits tend to sell the best when they are 'new' ahhh who doesn't LOVE that new kit smell LOL
I believe there are a lot of variables to the "worth your while" question.
-quality of product
-is there a market for your style
-advertising/marketing
I know there are many more but you get the idea

There is room for everyone and if you love what you are doing and have a passion for designing I would say go for it. On the other hand, there is much more to it than people think especially if you have your own store/site to maintain.
*ps sorry if i don't make sense but flat on my back and on meds from..
ah hem.. too much computer time