Or should I expect crowds to be smaller, and thus be able to buy on walk-up?
Yes, crowds will be smaller in November for the Eiffel Tower. The problem with purchasing advance tickets for winter months is that you might have tickets for a day which has zero visibility from the tower. I would just go early, about 09h30 when the Eiffel Tower opens and lines in November should be minimal.
I agree with Tocard, I would not pre buy as then you are stuck with that day and time and it’s not much fun if weather is bad , ie raining hard or foggy
We visited late September a number of years ago. We didn't have a problem with walk up tickets. Line to buy tickets and line to enter stairs were short. Line for the lift access was relatively speaking long.