1 Corr 15:12 = Now if we preach that the Messiah has been raised from the dead, how can some of you keep claiming there is no resurrection of the dead?
Presumably they were akin to the Sadducees, either Sadducee Christians, or converts inspired by Sadducee theology.
It's also worth pointing out that it's interesting that Paul's main argument to convince someone that Jesus really did rise from the dead is that all of them would be in real sh t if he hadn't: They would all "deserve more pity than any other people."
Actually, in this section he is arguing for the existence of the General Resurrection on the basis that Christ is risen from the dead. He's not trying to convince anyone of Christ's Resurrection here. Those he is writing to already believe that—what they deny is the General Resurrection.
He's also saying a second thing; that it is inconsistent to preach both Christ's resurrection (which they were) and to deny the General Resurrection (which they also where).
The one thing he isn't doing he is arguing for Christ's resurrection, because those he's writing to already believe in it.
You're artificially separating v.19 from it's context to make your point, in other words. You can't do that, it's a continuation of his thought from v.12 on.
1 Corinthians 15:12-19 RSV
12 Now if Christ is preached as raised from the dead, how can some of you say that there is no resurrection of the dead? 13 But if there is no resurrection of the dead, then Christ has not been raised; 14 if Christ has not been raised, then our preaching is in vain and your faith is in vain. 15 We are even found to be misrepresenting God, because we testified of God that he raised Christ, whom he did not raise if it is true that the dead are not raised. 16 For if the dead are not raised, then Christ has not been raised. 17 If Christ has not been raised, your faith is futile and you are still in your sins. 18 Then those also who have fallen asleep in Christ have perished. 19 If for this life only we have hoped in Christ, we are of all men most to be pitied.