No , you didn't miss anything - the example(s) were kept very simple for clarity.
For the example you are talking about , you would probably split the Rules into two Sets (.drl files) - one for sell and one for buy.
If your question is more 'how do I model more complex rules , you would either (a) make your preconditions more complex - more conditions to be satisified before the rule fires or (b) Arrange it so that Rule 2 would only fire of a change made to working memory by Rule 1.
The www.drools.org site has a lot of additional information on this subject.
Paul Browne - www.firstpartners.net/rp