Bruce has a good article that outlines the various business models you can use as a domainer:

1. High end speculator - buy a premium domain and sell at a much higher price. Rinse and repeat.

2. Hand registration flipper - buy reg fee domains and sell them quickly for $50 to $100.

3. Minisite flipper - put minisites on domains and flip them.

4. Passive income domainer - park your domains and live offer parking revenue. This is sweet stuff

5. Site developer - develop domains.

6. Domain collector - acquire large amounts of domains.

7. Expired traffic domain flipper - buy expired domains with traffic, build a site on them, and flip.

Why Do You Buy Domain Names? | BruceMarler.com

One thing he leaves out which is what I try to do is buy wholesale and sell retail. A few good enduser sales per year can pay for a good portfolio of domains.

I do a bit of all the other business models as well to a small or large degree. I'm more of a long term holder than a flipper though.

What sort of business models do you use and why?