Hi.
I'm think to transfer my main char which is 70 lvl ally to the same pvp server that i have a low lvl horde which are in the same account...

I have a second account available and i will transfer my ally there...

Having 2 dif account with 2 dif factions in the same server is legal.

I want to do this so i can trade through Booty Bay AH supplies (items, gold) to my low lvl char and i don't have to farm for 16 hours to get the 40 mount....

Is this legal or my char will be banned?

Two different accounts, you can have a horde character on one and an alliance character on the other, on the same realm.