IF it is new and unused in the box AND includes the receipt for warranty purposes then $90 really isn't that bad:
Canada Computers - Power Supplies/Management > Desktop Power Supplies : Corsair CMPSU-550VX 550W ATX 12V 41A 24PIN ATX Power Supply Active PFC 120MM Fan.
$92.99 + 13% taxes = $105.08. Forget the rebate, I never factor them in since they are always a 50/50 chance of actually getting them.
Besides Mike_E, it is RUDE AS HELL to chime in with crapola like that in someone's for sale thread. Try using the private messaging function if you really need to question someone's price. A seller can charge whatever the hell they want, you don't have to buy it.