* yeah, I do, now I think about it. It's because for some folks it's dress-up, not costume. Sometimes it's dress-up.
The only thing I bought on-line that needed a good fit (i.e. excluding stretchy dresses, hipscarves, veils, accessories etc) was a second hand costume on eBay. There were good pics of the seller dancing in it, info on how tall she is, her relevant measurements including regular bra size (in my book something vague like 34-36" C to DD means You Don't Know. 34G means you might have a clue ), and full measurements of the costume. It really needs to be that detailed for me to even consider.