The hardest part of buying second hand clothing is finding the right size. You might find the perfect vintage blouse, but there is no seeing it in a small. If it doesn't fit, it doesn't fit. On top of that, a lot of thrift shops don't have fitting rooms. And don't even think about taking something back if it doesn't look good on you. Returns and exchanges are almost unheard of.
So what can you do?
Well, I don't usually admit this because it is slightly embarrassing but, when I go to a thrift shop that doesn't have a dressing room, I wear a long-er loose-ish skirt so I can try pants and other skirts on under neath it. I know, I know. I have no shame.
If you don't feel comfortable trying stuff on in public like a crazy lady. Don't worry. Just get to know your measurements and keep a cloth tape measure in your purse. When you see that form fitting wiggle dress, do a quick measurement of the bust, waist and hips. You'll have a pretty good idea weather or not it will fit.
The final option is to alter the clothing to fit you. You don't have to be a seamstress. You can have your second hand stuff professionally tailored. If you pay an extra 20-30 bucks for alterations on a pair of pants that cost 5, you still paid less than you would for a new pair of new pants. But I don't recommend this unless you've found something that you absolutely can not live without. And remember, it's a lot easier to take things in than it is to let them out. So you should pass on stuff thats way too small.
There is a lack of size variety in vintage and second had clothing. It's sad, but true. Whatever you do don't get discouraged. The Gods of thrift will lead you to the clothes that you were meant to have. If it doesn't fit you, it simply wasn't meant to be.