first time registering you can't do it online.  to renew you can.  have to go to the dmv and do it in person first time.

says you just need a bill of sale and proff of sales tax paid.  unless its new then you need a mso.


ATVs, Snowmobiles-Required Documents
1.Bill of Sale (name and address of seller).
2.Proof of payment of sales tax (ST-6 or ST-6E). Note - To pay your sales tax online, visit the Department of Revenue Website.
3.Serial Number (ATV- 17 characters, found on the frame).
4.Required fees and payment method.
5.Properly completed and signed application  .

Please note -
•If the ATV or snowmobile is purchased new, a copy of the Manufacturer's Certificate of Origin (MSO) is required.
•Sales tax is due on the 20th day of the following month from date of purchase.
•Massachusetts sales tax is now payable online. To pay your sales tax online, visit the Department of Revenue Website or call the Department of Revenue at (800) 392-6089.


http://www.mass.gov/eea/grants-and-tech-assistance/enforcement/environmental-police/boat-rv-and-snowmobile-registration-bureau/