The best way to buy clothing is to buy them second-hand. A lot of people will not shop second-hand because they prefer their clothing brand new, but guess what? Once you've worn your new outfit once or twice it is now used! Since I've come to that realization it's been a lot easier for me to shop at second-hand clothing stores. And the money saved can be phenomenal.
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Sizes don't matter in clothing - just the fit. So learn to develop a sense of your size no matter what the size markings are.