I booked my flight with either Expedia or Opodo (have used both in the past, but can't remember which one was last year!) and while I could book my seats ahead, it wasn't straight away - I can't remember the timescales, but I'd phoned the airline to find out the date. I've just done a mock run through on Opodo for Air France flights from Edinburgh to Boston and I may be blind, but I don't see anywhere before the payment page that allows you to view which seats are available? I alwys buy my tickets and then try to book seats.
Thought the OP mentioned they were flying Delta but maybe that's a booking engine and not an airline??