That is a bummer. I really feel for you. When something like that happens you feel so deflated. A helpless feeling even though it is not your fault.
I'm sure you have read a lot on Marek's. Trust those articles from veterinarians and extension sites more than you do from a stranger over the internet like me.
I've never had Marek's in my flock. My father kept free-ranging chickens on a farm for over 60 years and never had Marek's. The disease is easily spread but it is not inevitable. Has anyone let that seller know they are spreading Marek's? I'd hope so. I don't know if somebody in county government the would be interested. If you are sure or even pretty sure You might call the county extension office and chat with them about your suspicions. They should know if anyone in government is interested.
Some general Marek's information. There are different strains of Marek's, different strains affect different body parts. Some are more deadly than others. Most of the time Marek's does not wipe out the entire flock, usually less than half show symptoms. But once it is in your flock every chicken in that flock is a carrier. Before I wiped out my entire flock and started over I'd talk to an expert about how well that works. Your county extension office should be able to help you with that at no cost to you. I don't think getting rid of it is that easy.
Flocks can develop a resistance to Marek's. That's different from immunity. If you carefully only breed birds that are not affected you will see a decline in how many come down with it. You will have to put down any that show symptoms, that can be rough. I don't think you will ever get a flock where none ever show symptoms but that number can be reduced, often dramatically after a few generations.
Obviously the part of rehoming chickens cannot happen but most of the other parts of your dream can still work. Marek's is not transmitted by hatching eggs. If you hatch chicks in an incubator and keep them isolated from your flock for weeks you can vaccinate them. They will still be carriers after they meet the rest of your flock but won't develop symptoms. If you let a broody hen hatch eggs the chicks will be infected as soon as they hatch so no use to vaccinate them. Some will make it but some probably won't. That's how you would develop flock resistance, exposing unvaccinated chicks to the disease.
There are different ways you can carry forward and salvage most of your plans but yes, life got harder for you.
