Hi,
They don't support the eBay counter, as Sellathon is part of Auctiva.
You could use the hidden Sellathon counter option (for tracking page hits) and use another counter service for unique visitors that automatically adds the counter with update to the listing at submission via their token (been there, done that).
Danno
Edit: There are some free services that also simply add a snipet of code to the listing before submission. Haven't tried those, but it's another possibility.