Hmm, well, that's tough. Alot of people, myself included, wouldn't pay nearly what this might be worth without some sort of certification that the signatures were real. Its too easy to sell forged stuff, and let me make it clear that I'm not accusing you of that at all. But. the certificate of authenticity is HUGE for me when buying signed stuff. It gives you someone to go after should you eventually find out its not real and helps you sell it later on. The purchase receipt, IMO, is almost useless, unless the company that sold it is still in business, well-known, and respected.
I have no idea what this might be worth. I'm more of a signed ball guy, not pictures. With no certificate, I might be willing to pay half of what it might be worth but no more. Its really a roll of the dice at that point and unless you got it certified, which can be done btw, the risk to the buyer is pretty great.
"You'll Never Walk Alone"