I know at least people who are successfully making a living selling - Markie's Mom of
http://www.marksfinestpapers.com/servlet/StoreFront
and Nichole Heady of
http://papertreyink.com/
Gina K too, of
http://www.shop.ginakdesigns.com/main.sc (but she also draws - that's not just stamping/scrapping, so dunno if that counts here). Of course, if you can draw too, you can sell your own line of actual stamps or digistamps.
Markie's Mom (aka Faith Hofrichter) and Nichole Heady both have a huge online presence with blogs with huge followings. They both know how to whip out awesome cards in just moments AND they know how to photograph those awesome cards brilliantly. Apparently, both are also awesomely talented at marketing, advertising, etc...the business end of things. They both also seem to delegate things - both have awesome design teams and other people doing other business things like shipping, website maintenance, etc.
If you have the talent and the knowledge (oh, and the ENERGY) to do this, or you are not afraid of delegating to others, I think you could do it.
If you want to make a living selling a direct-marketing company's products, I think the same applies - you'd have to have the ability to crank out awesome card after awesome card and know how to dangle those in front of potential customers to turn them into adoring loyal customers. Anything helps - a blog combined with awesome photography skills, um... well, that's all I can come up with, seeing as how I myself am NOT making a living doing any of this... *grin*
Make a Living Stamping???
