No...don't cull them all! Breed from those that are resistant!
And if you haven't done so yet, send the bird with the most obvious symptoms to an Ag lab for necropsy. A definite diagnosis would really help your business plans. There are other things that can mimic Marek's.
Yes, Cream Legbars are susceptible. The first breeding pairs (I think that Greenfire Farms brought over) all succumbed to Marek's in the US. Later stock was better, but the breed is known to have problems with it.
I lost a Cream Legbar x Barnevelder cross to what I am pretty sure was Marek's, though both hens that could have been her mother (CL's) never showed signs.
I'll link some information for you about how the industry is encouraging breeding for resistance rather than relying solely on the vaccine.
Informing buyers is the important thing. However, that also means being fully informed. Many buyers are aware that Marek's is part of the atmosphere, and all chickens are considered exposed. Many are interested in chicks from resistant flocks rather than chicks who have been vaccinated.
Personally, I would cull those who are severely affected. Clean your coops thoroughly. Watch for those who show no signs, and breed from them. Then sell the fertile eggs, which in themselves are profitable. (The cheapest I can find around here was $3 an egg, or $20 a dozen...better lines are $5 an egg).
The nice thing about selling fertilized eggs is there is less risk on your part, and no transmission of Marek's possible.
After your line settles, and you have no sign of Marek's for a generation, or two, then you can sell chicks...unless you decide to incubator hatch chicks and immediately vaccinate them for sale...keeping the unvaccinated chicks for the next generation of breeding, or you could decide to vaccinate all chicks (or eggs at day 18).
It is very frustrating...I know.
LofMc
https://www.pbs.org/newshour/science/tthis-chicken-vaccine-makes-virus-dangerous
http://www.merckvetmanual.com/poultry/neoplasms/marek’s-disease-in-poultry