That's what my instructor told me because you don't know what you are buying (the tree could be broken) and the leather quality is usually bad.
Personally I would buy a second hand leather saddle, because (i think) they look much nicer if you are considering showing. Go to a local tack shop or saddlery/saddler and ask for a 2nd hand saddle. If you arent sure how to fit it, get someone to do it for you, it is worth the money! I got a gorgeously comfy saddle for about 300 pounds (i can't remember) but its a brilliant saddle and has lasted well, it is an FB saddle but I can't remember what that stands for lol