Assuming you can get that to compile - it won't as shown - start with the debugger: put a breakpoint on the line
var obj = new List<Dealer>();
and look at the content of
param
to find out how many objects it contains.
At a guess, none. No objects, no
foreach
body execution, no objects added to the collection. But you can only confirm that with your code running with your data - so you need the debugger to confirm, and then use the stack trace to find out why. But even then, Concat does not modify the original collection, so you'd need to save the result to get anywhere.
Sorry, but we can't do that for you!