After years of shopping second hand I can pretty much eyeball an item and guess whether it will fit. Which really doesn’t have anything to do with the size on the label. Apart from discrepancies in sizing systems (Italian, French, EU, UK, USA) there are size inflation within systems. There is also a difference between designer sizing and mass market.
Vintage clothing often runs smalller in the waist than the modern equivalent.
An Italian 44 is about the same as a French 40, an American 6 or a UK 10!
Some French clothing goes by a 1,2,3 (T1, T2, T3) system.
International fast fashion brands like H&M often label according to more than one international standard – and these sizes are not correct. A USA 8 is closer to a European 40 than 38. If you are wondering which one accurately describes the size of the item, your guess is as good as mine.
UK sizes are very confusing to Americans because they run about two sizes bigger than the American equivalent. So a UK 10 is about a USA 6.
What to do? Forget the sizes and look for the fit. Find a jacket that fits you well and measure it and compare it to online listings or bring the tape to the thrift store. The most important fit points are shoulders. It is very difficult to change the shoulder width and it is the most important in terms of the overall fit of a jacket. On the other hand, hemlines are a simple fix if the length is too long. If it is short, check if there is extra material to let the hem out. Sometimes the creative use of another material, lace ribbon or even a scarf can lengthen a too short skirt or dress.