To my knowledge the only time the couple should buy the attire is if they are requiring a dress outside the budget range of the bridal party. My girls all bought their own dresses, but I let them pick something they liked that was within the price range they could afford.
I asked them each to be in my wedding. I also asked them each what they were comfortable with spending on a dress. I then made the decision to use www.azazie.com for their dresses. I simply asked each girl to choose a long dress in her preferred style in the color I chose.
Typically BMs pay for their own dress as long as you choose a dress within their budget. If you choose something outside their budget, you pay for it. If you are requiring certain accessories like shoes or the same hairstyle or makeup, you should cover that extra cost.
I just had a bridesmaid complain cause she had to buy the dress. She said in her words "I went out of my way to buy a dress for the wedding". I was just wondering if she was being unreasonable or I was.
It's very customary for the bridesmaids to buy their own dresses. Only one wedding I've been in was my bridesmaid dress purchased by the bride, but it was her gift to us and she didn't get the bridal party any other gifts.
I let my bridesmaids choose any dress they wanted in a variety of colors, even told them that if they had something in that color already, it was fine.
@OP I think your bridesmaid is the one being unreasonable, not you.