Scratch the morning: a lot of the walkups don't show up for work much before 11am - you could get lucky, but they'll be looking for the rush hour to start at noon. No bargaining then.
but they have a dead zone between about 2:30pm and 4:30pm, when they relax, eat a snack, and generally get bored. Get them in a good mood, everyone likes to haggle, and as long as they feel respected they might give you a deal.
For sure, you get longer and more relaxed sessions in the afternoon ...