Here's some background knowledge:
Each edition of Microsoft Office comes in two (or actually three) different licensing flavours: Retail/OEM and Volume License (VL).
VL licenses are for large companies and enterprises, but in the EU it is legal to sell, buy and use second hand VL licenses (although I do wonder where a second hand Office 2019 key should come from just a couple of weeks after the initial release...)
Since activation for the two licensing models works fundamentally different, MS distributes two different installation files. You can't use a Retail or OEM key to activate a VL installation, and you can't use a VL key to unlock a Retail/OEM installation.
MS only makes Retail/OEM installation images publicly available - no luck for VL users here. Installation media for VL licenses would have been given to the business that initially purchased it.
Office 2019 is also not yet available on MSDN (very strangely), therefore I can't find out the hash (checksum), which would make finding a genuine file quite easy.
Bottom line: there's not much I can do to help you. Better ask the seller for the installer.
(Reminder to all: it's not allowed to post copyrighted files or links to them here, just in case someone knows where to find it. Of course we respect users' privacy and don't monitor private messages.)