Because second hand shopping online involves finding an exact piece that meets your criteria rather than ordering a few sizes of the same item, it is especially helpful to think in terms of proportions.
Proportions are how much you see of one thing in relation to another. Look for the proportion you want and scale it to your own body!
Keep in mind the absolute measurements of a jacket you see online are not as important as the proportions of the jacket to the person who is wearing it. So if you see a blogger wearing a jacket you admire, check how long the jacket is on her (top of hip, top of leg, cropped to the waist) and measure yourself to the same point. That is the length you want no matter what the actual length of her jacket may be.
If, for example, you are interested in a second hand jacket that ends at the top of your leg, as mine does above, it is much more important to know your shoulder to hip measurement than the measurement of my jacket. Look for the proportion you want and scale it to your own body!