I certainly wouldn't want to bank on a "claim" by the vendor.
The best way to determine how many Dogs they are selling would be to simply count the buns that are being delivered in the morning vs. how many they are sending back.

Since you are there anyway, simply go past the cart after the buns hve been delivered.
Count them daly for a week and it will help get an idea of the number of dogs they are selling.
Granted there will be peaks and valleys in the numbers over a years time, however, if planned correctly, and given your marketing experience, you should turn larger numbers anyhow.

Get the wife to agree......
With your experience and track record, (between detailing, hotsheets, and everything else you have thrown at the wall) I would think that your wife would have grown to trust your intuition by now.

Best wishes,
And seriously Pete, if you can get the cart location, I will help you set it up with the locals, getting your license, your commissary, and other things needed to get started,
Again, best wishes,
Jason