Well I never did figure out how to do this, and AFAIK it is not possible.
What I ended up doing was to create a calculated field in my primary table, with the "calculation" merely being the content of the field I needed from the related table. Once I had that data in my primary table, I was able to then manipulate it the way I needed.
I'm posting what I ended up doing for the benefit of others who were stuck like I was.
I have not used FM10, but you should be able to use related fields in your layout. What table is the layout based on?
I am very familiar with eBay. You might think about upgrading to FM 14 from 10. You will be able to do so much more. Like manage all images, listing, messages, resolution center/refunds. Basically you can build you own custom solution for viewing and managing all your eBay accounts. Depending on your business model and products you can go straight from iPad to eBay very quickly.
You can also use FM to manage multi channel selling. As a side note eBay is not a very good platform these days for certain types of products. The fees from a business perspective are pretty high now, but it is still worth it to some people.
Thanks for the reply.
My layout is based on the eBay sales csv. I can display fields from my related (paypal) table okay - except for running totals. I could not get that to work. So what I did was pull in the paypal data into the eBay table for each record as the eBay data is imported. And that is working fine for me.
I'll have to consider upgrading to 14. I started with 10 because I had access to it. Currently I use Turbo Lister to manage my listings.
The categories that I mainly sell in dictate that I use eBay primarily as my selling channel for now. Up until about a month ago I was using a cumbersome spreadsheet for my bookkeeping. This is my first exposure to a database program and I am very impressed!