I recommend getting a new scooter with some warranty. The warranty is not really like in US or Europe, but if anything major go wrong it can be fixed (they put together Chinese bikes here in DR by kids and they hardly test it). There are many bike shops around DR, just try to find the fair price before going there.
Buying used could have some issues, you need to make sure the registration and paperwork are legit and also be able to check the bike mechanically - Dominicans could be very inventive to trick you in both areas. If you know and trust the owner then that's okay. Although to be fair even foreigners could be tricky (my own experience with an Italian), so shouldn't just blame the locals... !
Forgot to say, buy something that you see people drive around here, then it will be easy to get parts and repair...