Everything LazyGardener said above is spot on, that seller has no business selling birds. On the flip side, you bought birds from her before and your flock got sick after incubation, correct? In that case YOU also have micoplasma (or whichever upper respiratory disease you contracted from her). In that case, shouldn't cause any harm to your already infected flock to add new birds from same source. That said, a year is a long time, her birds could've picked up any number of other yuck if she's clearly that unabashed in selling contaminated birds to the public. There's a state vet in Missouri that estimates up to 90% of our backyard flocks are micoplasma contaminated (in mo).
There are a couple choices when you have reason to suspect your flock is contaminated.
1. Keep a closed flock, for the life of the birds, do not buy, do not sell. Once they have all lived out their lives and died follow proper guidelines to clean and start over.
2. Cull all birds on site, clean, start over.
In my opinion there's a third less acceptable option you could choose in this instance if you're attached to the flock. 3. Keep the flock, NEVER sell birds, and accept that you have an infected flock. Ad only birds like your chicks who are already infected (ms & mg pass through to the egg) or add healthy birds knowing full well that they'll get sick and need treatment after incubation (seems unethical to me, but chickens are one's property).
I've personally been through the devastation of this decision making, although mine ended up being a non lifelong upper respiratory illness. I loved my flock and decided to make sure before culling. I treated them, waited until they were all 100% and bought "tester birds". 6 months went by without so much as a sneeze or sniffle or runny eye from the testers. I naively decided my birds couldn't have had ms or mg.. but knowing what I know now about the health of chickens in my state, it could be more probable that the tester birds were already infected. I had to move, and sold those birds with utmost honesty: these birds have had upper respiratory problems and been treated for them, I do not have an official diagnosis from a veterinarian, blah, blah blah. It was an incredible flock of young beautiful birds. A man bought all of them for very cheap. Looking back it was probably very irresponsible of me to sell those birds, but at the time I truly believed them mg ms free. Maybe your seller does too? Hope my head doesn't end up on a pike.
Best wishes.
