import vs. include
The primary purpose of an import is to import a namespace. A more common use of the XSD import statement is to import a namespace which appears in another file. You might be gathering the namespace information from the file, but don't forget that it's the namespace that you're importing, not the file (don't confuse an import statement with an include statement).
Another area of confusion is how to specify the location or path of the included .xsd file: An XSD import statement has an optional attribute named schemaLocation but it is not necessary if the namespace of the import statement is at the same location (in the same file) as the import statement itself.
When you do chose to use an external .xsd file for your WSDL, the schemaLocation attribute becomes necessary. Be very sure that the namespace you use in the import statement is the same as the targetNamespace of the schema you are importing. That is, all 3 occurrences must be identical:
WSDL:
xs:import namespace="urn:listing3" schemaLocation="listing3.xsd"/>
XSD:
<xsd:schema targetNamespace="urn:listing3"
xmlns:xsd="http://www.w3.org/2001/XMLSchema">
Another approach to letting know the WSDL about the XSD is through Maven's pom.xml:
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>xmlbeans-maven-plugin</artifactId>
<executions>
<execution>
<id>generate-sources-xmlbeans</id>
<phase>generate-sources</phase>
<goals>
<goal>xmlbeans</goal>
</goals>
</execution>
</executions>
<version>2.3.3</version>
<inherited>true</inherited>
<configuration>
<schemaDirectory>${basedir}/src/main/xsd</schemaDirectory>
</configuration>
</plugin>
You can read more on this in this great IBM article. It has typos such as xsd:import instead of xs:import but otherwise it's fine.