>>20120458>>20120487;_; has always been acceptable when spoilered and even unspoilered isn't reaction worthy.
>>20120404general rule of thumb non-event legendaries are worth other non event legendaries. Older and competative viable legendaries are worth more.
Personally I don't trade my shinies but you can sometimes find people tradeing theirs for a non-event legendary so it goes
event legendary > kalos born shiny > pre kalos shiny > non-event legendary > hacked shiny/legendary
This is basic things like clonedmons, hackedmons, non kalos born mons, shiny legendaries, and certain eventmons all may be worth more or less depending on who you are dealing with. Plus keep in mind a lot of kids play this game and will not follow any rule just trade for what they want.