i've got a full shop sync software as well which syncs MS Sql based database (Helios stock accounting and stock system) running on remote server which syncs all the products, categories, customfields, images, prices, supplier databases with the remote system daily or on demand via background CLI jobs.
this type of custom sync system is usually a matter of hundreds of hours to get it done properly and effeciently and the system needs to handle addition/update/removal of all related items.
my system doesn't run on joomla or VM, but inserts the data directly into the database and expects a dedicated servers on both ends with plenty of RAM, system is optimized for high latency internet lines where queries might be returned in tens of seconds from the remote server.
another sync software which is part of our OPC syncs Sherpann.nl stock system (VM to Sherpa products, Sherpa to VM stock + invoices, VM to Sherpa orders ), it runs on PHP-CLI and loads VM and Joomla during the process.
best regards, stan