AWPCP supports the notion of searching for ads based on various criteria. This is done through the Search Ads page, where the standard search fields are present:
You can control which fields are visible in search under Form Fields settings. Also, if you have the Extra Fields module installed, you can display search fields for your extra fields defined in ads. For example, you might have a Make, Model and Year field added for a Car classifieds site. AWPCP Extra Fields would allow you to automatically expose these fields on your Search Ads page so that people can view ads based on those fields.
Clicking on Start Search will take you to the Browse Ads page, filtered for the results you asked for in Search Ads.