Sync with third-party vendor's product files

One of the major issues we have been having is, importing and working with third-party vendor's csv files to be shown as Magento products. This information must be dynamic and bi-directional, meaning, every time we sell on our system, it should update the csv file and update their (the third-party vendor's) system. And vice versa, when a vendor sells out of a product on their end, they should be able to update that inventory on our end (on the live Website) via the same dynamic spreadsheet.  Some companies use a xml feed file and update on both ends (third party vendor update and retailer update).