I have a large stock of 460.000 Listings on ebay, so the syncing process to migrate from inkfrog to Market Place Connect fails, because the syncing process is in an endless loop.
Anybody has many Listings running on ebay and uses Marketplace Connect.
Honest reality check though, at your size you might be better off looking at CedCommerce or Codisto LINQ instead. Both handle high SKU counts more reliably than Marketplace Connect right now. Worth demoing them before sinking more time into fighting the sync.
I can’t comment on CedCommerce or Codisto, and my store have a much smaller inventory, but we’re finding LitCommerce great for their eBay integration. It might be worth reaching out to them, to see if they can handle that number of listings.