1 y/o hen died of Marek’s. Should I sell my unintroduced pullets to save them?

For the future, only buy vaccinated chicks, and isolate them for two to three weeks, so they build up immunity to the tumors, before introducing them to your flock. It would be best to have them off site, not at your place at all, during that time.
It's the tumors that kill infected birds, and the vaccine helps to prevent the tumors from developing.
If you raise your own chicks, expect to have deaths, while you try to produce a flock who has some resistance to the strain of Marek's on your property.
And, never sell birds, or send them elsewhere. and practice biosecurity to protect others from this disease!
This is why I never bring in birds from outside!
Mary