It wasn't really an option; when states passed gradual emancipation acts, they also passed laws that made it illegal to take soon-to-be-freed people out of state for sale. This actually was in an issue for none other than Sojourner Truth whose son was illegally sold out of NY state before emancipation and who had to track him down and sue to regain her son's freedom. But by the time slavery was abolished in the North, slaveholdings were generally small and gradual emancipation meant enough time (these bills made slavery expire years or decades in the future) for people to gain most of the 'value' of their human property. It wasn't worth uprooting yourself and breaking the law to preserve holdings of 1-5 people...