Whom did the Lord use besides Peter and the eleven apostles?
Besides Peter and the Eleven, the Lord used Paul, Barnabas, and later Silas. Paul points to himself as proof of apostleship, explaining that the Lord called and used him along with Barnabas, and afterward Silas, demonstrating that the Lord's work was not limited to the original Twelve.
Les explains that Paul was accused of not being a true apostle because he had not spent three years with Jesus as the Twelve had. Paul answers that the Corinthians themselves are the proof of his apostleship, and it was not just Peter and the Eleven whom the Lord used. The Lord was using Paul, Barnabas, and a little later would use Silas. Paul is still dealing with his apostleship in this section, showing that a servant of God is worthy of his labor, whether pastor, teacher, or evangelist.
If I be not an apostle unto others, yet doubtless I am to you: for the seal of mine apostleship are ye in the Lord.