I'm looking for something like either an API that will let you query a catalog of products by category (for example trousers or shirts) and get comprehensive product information back or this same thing in the form of an offline dataset. The vendors doesn't matter but it has to be in fashion.
Have you considered tying together a couple of Ebay's Buy API endpoints like the Taxonomy API to get the category id combined with their "Browse" endpoint?
Taxonomy Endpoint :
Browse Endpoint Example (using categoryID):