I haven't seen the feature version of Cashback. If a remember rightly (I was only watching for one reason), the short is about a guy who freezes time in a supermarket, then uses the time to draw the patrons nude. I believe the feature uses this exact scene as a kind-of 'dream sequence'. Saw was made as its own story, but is essentially the story of Amanda (the girl in the reverse bear-trap and eventual killer), only in the short it's a man (Leigh Whannel, writer and actor from the feature).
Nick's right though. A trailer may show an investor that the film you want to make could be good, but that won't be enough to sell it. I only suggest it as I think it would be a lot of fun to do, but, realistically, it won't get you very far.
Am I wrong, or did I not read a while back that you had a fair bit of money to fund your film yourself? Why are you trying to seek investment now?