Yep, it helps to have the confidence to say no to an unhealthy bird.
One time I bought chickens from CL that were probably older than they said and in rough shape. I didn't have the will to say no after driving out all that way. They were fine but didn't lay well and I should have said no.
One...
People also sell not sick birds on CL. I sell all mine through CL - BYC is too unreliable because people are from all over so who knows if someone in your area wants your birds.
Just gotta have a good quarantine. And maybe a good eye.
I'm not seeing a roo and checking egg fertilization through the blastodisc is notoriously unreliable.
But feel free to get individual shots. At 8 months old it should be pretty obvious if you have a roo.