First I'll state what I do, and then add in a bit of advice in the next paragraph.Vaccination creates weak stock. If people who approach me about buying birds ask me, I tell them I don't vaccinate (or medicate for that matter) for anything and why. If they're too soft to handle things that way in my view they are too soft to be a good steward of the breed and I would prefer the birds go to someone who understands that. Better to lose a sale then to see the birds in uneducated hands. Like you said, the strong survive. Marek's is everywhere, it's just the way it is.
Breed your birds the best you can, vaccinate if you really feel the need but recognize that you're weakening your stock. Which could also reflect poorly on you if they buy stock, breed it and then have unhealthy offspring. If you keep your stock strong, people who buy from you will be able to breed strong healthy stock on their own and be happy they came to you.