Nope... blockchain.info allows sending from multiple addresses in one transaction.
Nope...is in response to what?
most someone can do is rip you off for the value of one bill
You can rip someone off at every purchase... hell if I would ever accept these.
The ripoff opportunity is the other way around, the person presenting the bill gets ripped off, not the one accepting the bill.
The one accepting the bill merely scans the bill and initiates a "sweep" transaction to immediately send the funds to another address. Unless a conflicting transaction shows up on the network immediately, it's pretty safe for the seller.
Back to the OP, I think they are a worthless idea with very little real world application. Selling them for PayPal is an open invitation to be ripped off, setting up a system where you can be ripped off so easily will make people want to reverse these paper bitcoins, having reversible paper bitcoins floating around does nothing to enhance the value of any other bitcoin, and leaves a bad taste with anyone who touches them.
And I used to do the exact same thing, I sold paper wallets for PayPal (notice paperbitcoinwallet.com forwards to casascius.com). No one ever ripped me off, but there isn't a whole lot of demand for them either, not when you can just go print your own for free and not worry someone else kept the private key and is going to steal your money in the future. My purpose in doing so back in the day was not to make money, but simply to promote the fact that bitcoins could be kept on paper when very few realized it was possible and before bitaddress.org came to exist. People understand this now, mission accomplished, so I don't bother to sell them anymore.