you're only hurting yourself if you take a solid idea and don't try shopping it around before selling it yourself. Unless you're starting a full personal storefront with real, quality screenprinting.
I can't compete on this site, and I"ve tried some other designs on threadless that haven't worked. I think I'm a fairly competent designer, and I've come to realize that good design doesn't sell a shirt. Woot demonstrates this wonderfully every week.
I have a businessman friend who I will be working with, we are going to build a full storefront, and we are currently looking for a print shop to work with.
With any luck, I can get a customer following.