Hi,
Sadly TaxJar has recently changed their pricing model such that this plugin is no longer usable by anyone who doesn’t subscribe to their “TaxJar Plus” plan. As a result we will be discontinuing support for Marketplace Taxes in the coming weeks. Sorry!
– Brett
Sorry for the delayed response.
Unfortunately YITH Multi Vendor compatibility is not on the road map at the moment; however, if we get more requests we’ll make it a priority. My apologies for any inconvenience.