I have Payment, Shipping, About Us, and Contact Us seller details all in Verdana Font size 3. However, for some reason, the Shipping is displayed as Arial font size 3, even though it shows correctly when I'm editing the seller details.

Here's one of my auctions for an example:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=130086680550

This has to be a bug with Auctiva's Seller Details because even the text editor displays the Verdana font tag on Shipping.

Does anyone know about this issue?

I had that same problem. What I did was go into the description under the HTML format and read in there if it says Arial font size 3, if so, just delete that part only and save it. Then you can go back to the design editer mode and set it to how you want it. Hope that helps, it worked for me. Also applies to colors, etc. Or, create a whole new profile.