You risk it breaking down soon after purchase and there's nothing you can do unless it's still in warranty and they will give you the receipt.
If you have $300 you only need $100 more, I see a 40gb PS3 is only $400 on Amazon.com.
Maybe you have a few games or gadgets you could sell on Ebay to make the extra $100.
At least then you will have piece of mind for 1 year whilst it's in warranty.
Oh yea, and use a credit card if you can, even on Ebay, because that gives you extra cover on expensive items.