I wouldn't accept any offer for it until you can get a full understanding of it's value. Take your time, the offers may just climb! My suggestion would be to private message mike diamond about it. He runs CONECA (http://hermes.csd.net/~coneca/). Once again, approach the offers with caution!
It's a grease strike. Value would probably be between $20 and $25. And that's only because it's a Union Shield cent.