Firstly, you are offering a common, easily available Pokemon for a rare legendary Pokemon. That is not an equal trade and thus there is no reason for the owner of a rare Pokemon to want to trade for a common Pokemon. If you really want that Shaymin, you must offer an equally rare Pokemon such as another Legendary Pokemon, preferably one that is obtained by event only just like Shaymin. Or you could hack Shaymin by various methods, which should be described elsewhere online. Personally, I don't collect Legendary Pokemon because I can't use them in the normal gameplay and they are more like trophies. They might complete your Pokedex but I play Pokemon because I like the gameplay, not because I like collecting achievements.