You say only sold where legal? But they are definitely not legal in Massachusetts.
They definitely are legal in Massachusetts. Aside from a small list, all turtles are legal except those Federally or state listed as endangered:
1. Species categorically non-exempt pursuant to 321 CMR 9.01(3).
2. Argentina or Chaco Tortoise (Geochelone chilensis).
3. Gopher Tortoises (Gopherus polyphemus) and all other species in the genus Gopherus.
4. Pancake Tortoise (Malacochersus tornieri).
5. Spotted Turtle (Clemmys guttata).
6. Red-eared Slider (Trachemys scripta, including all subspecies and forms.)
9.01(3):
(3) Categorical Non-exemptions: No species in any of the following categories may be exempted:
(a) Threatened or endangered species listed pursuant to the U.S. Endangered Species Act of 1973, as amended;
(b) Species listed in the Red Book(-s) of the International Union for the Conservation of Nature;
(c) Species appearing on the Massachusetts list of endangered, threatened, and special concern species at 321 CMR 10.90.
Turtles listed on 10.90:
Loggerhead Seaturtle Caretta caretta T T
Green Seaturtle Chelonia mydas T T
Hawksbill Seaturtle Eretmochelys imbricata E E
Kemps’s Ridley Seaturtle Lepidochelys kempii E E
Leatherback Seaturtle Dermochelys coriacea E E
Wood Turtle Glyptemys insculpta SC
Bog Turtle Glyptemys muhlenbergii E T
Blanding's Turtle Emydoidea blandingii T
Diamond-backed Terrapin Malaclemys terrapin T
Northern Red-bellied Cooter Pseudemys rubriventris E E 4
Eastern Box Turtle Terrapene carolina SC
Alligator snapping turtles do not fall under any of these exemptions/regulations. All covered under Massachusetts 321 CMR 9 and 10.