He was busy persecuting them:robtex said:I find great interest in that verse too. Could you please list names of some of the 500 (with sources of names attached) and explain why not a single one of them came forward to colloborate Paul's claim.
Furthermore, you do realize that Paul said he did not see the resurrection himself and converted on the road to Damacus (acts 9 9) at an unspecified later time when he claimed to have been visted by Jesus. Why do you think Paul did not convert at the time the 500 witness told him of this amazing endeavor but than turned around and assigned merit to it?
Galatians 1:13 = FOR YE HAVE HEARD OF MY CONVERSATION IN TIME PAST IN THE JEW'S RELIGION, HOW THAT BEYOND MEASURE I PERSECUTED THE CHURCH OF GOD, AND WASTED IT.
He then, later, meets Jesus' Himself while he's traveling to Damascus, and gets saved.
It's possible that the 500 people he's talking about are some of the Christians he, himself put to death. Stephen being the most famous one in Acts 7. (See Acts 8:1, where Saul was present at Stephen's execution.)