Do be very careful if you choose to do that. Often any genuine used copies will have had their serials already registered, so you won't be able to get updates or play on ICO unless the seller also hands over his accounts. Counterfeit "foreign import" versions are common on ebay as well, though thankfully I've not seen that yet with Sins specifically.
Well got the game and all. Works like a charm and i have registered it and all. Oh and cheers Kryo for your help in my thread.