I'd need to see one before you make a fix at eBay for any possible suggestion on cause.
You could use the HTML export on the Saved Listing and copy - paste the HTML to www.practicewriter.com for one that produced a problem. Make sure you "comment" on what's wrong with fonts on any post with a practicewriter posted link.
That will get me and the Auctiva folks "looking".

Danno
Edit: Are you consistently using the same Auctiva template, or different ones? And, are the problems assoicated with use of a specific template?