Why is sizing so chaotic, especially with second-hand clothes?

I just bought something that was listed as one size and when it came… it was not that size at all. Proper “what I ordered vs what I got” moment. First time it’s happened to me and it’s so annoying.

When you guys buy clothes online, how do you pick your size? Do you actually check measurements or just go with your usual and hope?

How often does stuff end up fitting wrong? And what do you wish sellers would include but usually don’t?

Genuinely curious because this has put me off a bit.