It is hard to describe what you are asking.<p>1-) You must solve a problem of a niche industry.
2-) You should do it very simply because you are one man operation. Don't add too many functions / options so you can maintain it until you grow your "one man" operation.
3-) Pick an industry which won't change very fast so you can actually market your product.<p>Other than these 3 common things, I can't think of anything until you describe your product a little.