I understand the value of experience. I understand why he tried to use his experience to raise his soapbox as having “added value” to his testimony.
But testifying is what he was essentially doing. He was testifying that “this is what conmen do”. He was “selling us” his perspective, through that testimony. The testimony of a self-professed liar is pretty useless.
A better analogy is somebody saying this:
So you want to buy a timeshare in Orlando? Great. But be careful, every one of those timeshare salesmen are full of crap. They will rip you off. I know, because I made millions ripping off people for years as a timeshare salesman. So I know how corrupt and sneaky they can be. I was the king of ripping people off. Give me your money and I will take care of you, so they do not screw you over.
Would you buy anything from him?