It's harder with second-hand sales, but there are some brands whose sizing is more consistent than others. For example, DD1 is a string bean (always was). Carters onesies were thin and long, so they fit her better than, say, Cherokee which were wider and shorter. Once we found a brand whose shape fit the kid well, we tried to stick with it.
Now that there are two girls running around the house, the clothes sometimes get mixed up. I've looked at the 18 mo old's pants and thought "huh, those are cut a little weird", only to find they were her sister's size 5 shorts (with the waist waaaaaaay cinched in). I think she was wearing her sister's size 3T leggings the other day, and they fit her just fine.
You've probably already thought of this, but bring an existing shirt/pants/shoe with you to the sale so you can measure the new things you'd like to buy against clothing you already know fits, or is going to be small.