If I want to create a site similiar to the craigslist.com, contents will be filtered first by location, then category. The user case will be:
1. A user first clicks on location, that could be region, then city.
2. The user clicks on category.
Step 1 and 2 could be reversed.

What is the best way to do ?
1. Where can I save the first filter when the second filter is clicked ?
2. Assume that two blocks will be used, one for location, and the other for category. Can the default taxonomy system with menu be used to build the block ?
3. Any example module that I can follow ?

Thanks for your help in advance !
David