Cars for sale section, here:
VW TDI/Diesel cars for sale and wanted.
forums.tdiclub.com
Others have had success with Craigslist and Facebook marketplace. There are many other, more car specific options, as well.
One of my frustrations when looking is how fragmented it's become and needing to check multiple places to find tdis.
Once you have an ad you are happy with, probably best practice is to post it here, Craigslist and Facebook as a minimum. Other's may have additional places they have had better luck using.
You may want to make one your "primary", say Craigslist, and just link the other's to that ad. That way you can just make updates in one place.
GLWS