Even though Paul had approval from the eyewitnesses of Jesus' life, nevertheless, Paul claimed that his ultimate authority did not come from humans, but directly from Christ's revelation to him:<quoted text>
Paul/Saul of Tarsus ... we know he never met the Christ Character because he told us so. He says Christ came to him in "visions". I have frequent "visions" of Jessica Alba, but I'm quite certain she has no idea who I am.
"For I would have you know, brethren, that the gospel which was preached by me is not according to man. 12For I neither received it from man, nor was I taught it, but I received it through a revelation of Jesus Christ (Gal. 1:11-12)."
Paul himself saw Jesus via revelation on his encounter on the road to Damascus (Acts 9, 22, 26) in which Christ revealed Himself to Paul . He was an eyewitness in the sense of having seen Jesus after his death (1 Cor. 9:1). Therefore, Paul certainly had the authority to teach and preach the Gospel.