Living in Massachusetts - just bought a car, to which the dealer still holds title. Can the vehicle be seized by the Constable's office to satisfy a court judgement for credit card debt?

If the car is not yet titled in your name, it is not likely, they would have to prove your intent to hide the car by not transferring the title.
** if this is a car you have not yet transferred the title. IF for example this is a buy here pay here type of place and they keep the car in their name, till you pay most of the car off, there is no record of you owning the car, so they could not find it first of all.