Marek's isn't the only virus. Others have no vaccination, no cures. My flock carries one of these, worse than Marek's, and the heart ache is indescribable.
Return the chick if this breeder is not NPIP certified.
Was this breeder a private breeder? If so, are they NPIP certified? If you don't know, call them and ask. It's a program that inspects and certifies poultry breeders and hatcheries to insure serious poultry disease isn't spread around the country.
This certification should give you pretty good...