I used to go to vintage markets to buy clothes and I always thought you should wait until the end when retailers want to get rid of stock and you can haggle. For vintage clothes though, you go early as many people go to buy for shops and sell them on. I did this once and learned to ask for the best price directly and usually got a discount when they knew I was serious and not browsing. There is another side, if you're not sure, you can keep popping back to look at it and you may get a discount, but don't go too low if you try to offer because insulting the stall owner can end up with them blanking you out.
One tip I can say is to buy many clothes at once. It may sound crazy but haggling is easy this way as the owner may give you a bigger discount if you buy a lot. Think of wholesale bulk discounts, that is the same way as this!
When I purchase alot I ask for a discount also another thing to remember to ask the owner is what day do you offer a discount? The owner may not have a day to offer a discount but now you have given them they hint that you are going to purchase a lot and they are more open to giving a discount.
According to me the best time to go for shopping or buying an item is the time when you need it. I always buy or go for shopping when I need an item(s) or I have really good savvy option (coupon,deal).
For individual sellers selling vintage clothes, the best time to purchase would be right after they refresh their stock since you could get better items for almost the same amount. Bargain markets usually already have very low prices so it wouldn't be very efficient to wait for their off season as you would at an outlet store, in my opinion. The best tactic for me personally is to get to know the sellers so they give you an inside scoop on when it is best to buy from them, usually when they refresh their stocks.
I find that the best time to get a bargain is right after holidays. It's typically when stores are looking to clear out their merchandise and make room for the new. As it is after July 4th, a lot of U.S. stores right now are featuring up to 70% off clearance. Also, it helps to ask at the stores if you can sign up for their rewards program/credit card (though this is not always advisable) to get a certain percentage off of your purchase. However, not every store obviously offers this.
I have to agree with you or before a holiday. That's when a store normally has their sales because they know that they will get more people to come to their stores then. Like you said, they are trying to get rid of all the old clothing that they had since they assume that they will be getting more. Black Friday is one of my favorite times to buy clothing just because of how cheap you can find clothing. I find that you can get the best deals then.
