The problem is figuring out which model. These are the most common models seen in this country.
They started producing these guns in the mid-1920's until sometime in the 1960's. If it is not one of these get some pictures.
There 500 years worth of Beretta's out there.
Notice the differences between the 401 BIS and the 401 Vittoria.

Check the proof for the date code.

Then depending on the condition and gauge you can begin to determine a value.
It sounds like this dealer is trying figure out the same thing.
They obviously know what the gun cost them.
I recently missed a 403 Stella in 28 gauge, the asking price was $2,500 in 90% condition.
Pete