For example, let's see how the New Trade filter works. The concept is the same for all other pages.
Selecting filter's list fields.

The filter is located on the game list panel. For each filter you can select several items. You can find the filter's value by typing its first letters.

There will only be those items that satisfy all the filter fields combined.

For example, if you have choosen "Factory New" and "Well-Worn" filters for Exterior field and "AK-47" filter for Weapon field you will get every AK-47 with Factory New or Well-Worn qualities.

Deselecting filter's list fields.

To delete the filter just click on top of it. For search field clear its field.

The search field.

To find a specific item, you must enter part of the word or the whole word, which is in its name. You can also enter more than one word to narrow the search results. The order of words in the search field does not matter!