Choosing Consecutive Seats by Seatrow
If you're happy with what you have already, try starting with a set of the distinct sections, cross joining on that, and then filtering your subquery to only that section. That way, it's like considering each section as if it's its own event. 

But... why not consider looking at all the seats and finding blocks where the (N-1)th next one is the actual right number away:

SELECT * FROM
(SELECT seats.*, eventtickets.eticketprice,
    LEAD(seats.SeatID,@N-1) OVER (PARTITION BY seats.seatSection ORDER BY seats.SeatID) AS SeatNAway
from seats 
INNER JOIN eventtickets ON eventtickets.etickettype = seats.seattype 
WHERE ((Seats.seatType ='BOXSEAT') 
AND seatID not in (select seatID from ticketsales WHERE eventID=6))
) s
WHERE s.SeatId = s.SeatNAway - (@N-1)
ORDER BY convert (int, s.seatSection), s.seatrow ASC


See how you go with this maybe... check that you understand the inner subquery, of course.