How do I retrieve a list of available folders and thier id's for a user account?

In order to retrieve a list of available folders and their IDs for a user account, make a call to GetMyMessages with DetailLevel set to 'ReturnSummary'.

Here a code sample from the documentation -

<?xml version="1.0" encoding="utf-8"?>
<GetMyMessagesRequest xmlns="urn:ebay:apis:eBLBaseComponents">
<RequesterCredentials>
<eBayAuthToken>TOKEN</eBayAuthToken>
</RequesterCredentials>
<ErrorLanguage>en_US</ErrorLanguage>
<WarningLevel>High</WarningLevel>
<DetailLevel>ReturnSummary</DetailLevel>
</GetMyMessagesRequest>