I also sell mostly patterns. Like Crystal said, it's not an easy craft. But my professional background is in technical communications and I get jazzed about that kind of stuff so...
Here are the Pro's and Con's -- the way I see them:
PRO - Patterns sell over and over (and over) again. Once you have a good one, you can re-sell it as much as you want. And, it requires little to no effort to do so.
PRO - Patterns lend credibility to your design. We all know that amigurumi is hard work, but a lot of buyers don't. If customers see how many stitches go into a single piece, they start to understand why we're asking certain prices for pieces.
CON - Transitioning from a customer base that buys mostly dolls to one that buys mostly patterns DOES NOT happen overnight. It took a good year of careful attention and marketing to make the transition with The Itsy Bitsy Spider -- and I still get a LOT of requests for finished versions of my patterns.
CON - Copyright infringement. Yup. If you put your patterns out there, people are gonna steal 'em. Plain and simple. Decide up front how you want to handle this. My policy is to allow sales of finished items from my patterns as long as they are not mass-produced. It saves me a lot of time and angst in chasing after mom & pop crocheters who just want to use the dolls for the church carnival or or earn back their yarn costs.
CON - It's not easy to do a pattern right. I've been at the pattern thing for about two years now and I still learn something new every day.
CON - You are responsible for pattern errata and numerous numerous questions. Even if you say that the pattern is for intermediates, you are bound to get a few beginners who want to try it out. How do you plan to support the ranging skills of people who use your patterns? What will you do if a mistake is found?
As Crystal said, pattern s are a LOT of work, but I think the work is well worth it. If there is anything I can do to help you get started or any other questions I can answer, please feel free to contact me!