Need help for integrating inventory management system with auction sites

Hi guys..

I am currently required to develop an inventory management system for used cars that posts and retrieves data from single/multiple auction sites. The inventory management system should be standalone and it can integrate with the existing auction sites like eBay etc. or for demonstration purposes, can be shown on a separate new auction site. Can you guys please help me as to where to start with this and if there are any ready-made scripts available that suit this purpose? Please help

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