How weird is it to have a guide on this ?well,I have bought over 90 firefighter tees from different sellers on ebay and I'm glad to say most of them are exactly what I expected...
however,a good rule of thumb is naturally check the size...particuarly american or FDNY  job shirts which tend to be one size bigger when you get them,ie if you take a large buy a medium...
Also on the T shirts the crest on the front should be around three inches wide on the wearers left,a couple of the generic shirts I got had a crest like a saucer that looked bollix when it arrived..it didn't help that the image was digital on the listing and not a photograph as you had no way of telling how big the crest was on the chest as you would with a photo...
also shipping seems to vary across the board..In my experience expect to pay no more than approx $12 dollars states to europe..anymore is generally a racket...
Trying to get genuine apparel can be difficult on ebay and I was lucky enough to add a couple of FDNY polos and EMT polos to my collection as well as one of the navy shirts