Like wolfeuk said, signs have nothing to do with trading. What a lot of people do is put signs next to villagers to add some cosmetics to trading but the signs don't do anything to trading. Just left trigger on a villager and it opens up the trading GUI... vuala.
Things like that do not exist on the Xbox 360 Edition. The Xbox 360 Edition only has whatever is in the regular unmodded Minecraft released by Mojang.