There are no photos of details in this post so impossible to say what the value is, $500 would be a very good price though for a scarce rifled version of the M1842. That is IF it is a "buy it now" price. I suspect it is an auction with a low starting price and will sell for considerably more, even with the missing or sheared off tumbler screw.
As far as the bore's condition? As Flayderman says, the bore condition does not have a lot to do with the value of an antique gun. Most antiques are not shot, especially those that are hard to find.