The PSU is made using some Fuhjyyu (Chinese word for feces and a capacitor brand... extremely low quality) capacitors and cannot deliver its maximum rated wattage without exploding and damaging components. Most shop owners give low quality pieces to customers and say that they have one and they are great, etc..... It's just quanity/quality for the shop owners like that.
Actually I didn't buy the PSU at his store, That PSU was just "reccommended" by him. BTW how about that silver stone PSU