Those 2 that he now has pictured in his ad are the biggest and healthiest that were in his run by far, you know what your chickens look like if when you hang out with them everyday, lol (no life).It does look like a barred rock to me, too. I think the seller was wrong the first time when he sold you this chicken as a sex link. Was it a chick when you first bought it? Also, I looked back at the picture you posted, and your rooster at 20 wks looks a lot younger than the one in the craigslist ad. Maybe it's a different rooster.
I was told at time of purchase he was 5 weeks; this was the day we brought them home.
This was him about 4 weeks ago;
And now you see what he looked like last week. It's the old "If I knew then, what I know now" things... When I bought the original 5, he gave me a breed sheet that either he printed out or he got from his breeder that described the 5 breeds that he was selling. He didn't have any Barred Rocks and there were none listed on his sheet, or that is what I would have bought.