Ermenegildo Zegna thrifted from ebay.de
Find a jacket of yours that fits well, measure it, then try to find something with similar measurements.
You need to know what measurements fit you well. As was mentioned, measuring a coat or trousers that fit you well, and then looking for something that is close to that, will help a lot. Be advised, nearly everything I have gotten from ebay still requires some altering by a tailor to fit well. Even then, sometimes you will get something that just doesn't fit and there is no way to alter it. That just happens. Always be sure to check the seller's return policy. I have returned a great many things over the years. Once you get the hang of what fits you, buying on ebay can be fun and affordable.
The problem in my experience with buying suit or sport coats on ebay is that it's not sufficient to know your own ideal measurements, it's also necessary that the seller know how to measure properly. A surprisingly percentage don't.
Seller feedback is very useful in such circumstances, but of course it's not infallible.
Before I became an old vintage man I used to wear black or white shirts without a traditional collar. In the neck I had a silver button, one with a small diamond from my wifes earclip and one with black enamel. Today I tried them on for the first time since october 2010. The shirts was much too big, so I have to order smaller shirts to use these buttons again.