All Paul is doing is debunking the belief that sprouted up among the Christians back then that there is no resurrection of the dead with the logic that they already knew for a fact that Jesus was resurrected from the dead and that was a basis for faith in the resurrection of others too. In fact, the resurrection of Jesus is what they were preaching about.
Slow down there, turbo-dog. Let's not get ahead of ourselves. Where did Paul say in that passage that they knew for a fact Jesus had been raised?
(These guys back then weren't teaching from some book they read which was written some thousands of years ago. ( The Bible so states that:)
Well, books didn't exist back then, so of course they weren't. However, they were preaching from scrolls and teachings from thousands of years ago.
They were testifying about what they saw and heard-not hearsay or something that they read off some book, and that is what being a witness is all about.
Paul never met Jesus when he was on Earth, even by the time the Gospels were written, most of the followers of Jesus had never seen nor heard the man.
Facts cannot be invalidated; neither with logic, not with arguments, not with other facts. Facts stand on their own and everyone back then was convinced for a fact that Jesus rose from the dead and no one could prove to them otherwise. And that proof was a basis for faith in the resurrection -not of Jesus- but of everybody else.
Being convinced of something doesn't make that thing a fact, nor is it proof of anything.