As you've already discovered, trailers over 3,000 pounds require Titling in Massachusetts (See below references).
Title requirements, here quoting from
this link:
State of Massachusetts wrote:
Section 4. (a) Except for vehicles referred to in section two, whoever acquires a motor vehicle or trailer after the effective date of this chapter, shall be required to make application for a certificate of title. Such application shall be made within ten days from the acquisition of ownership of said vehicle or trailer.
There is a trailer-related exemption from this requirement in the section (2) mentioned above, here quoting from
this page:
State of wrote:
Section 2. (a) No certificate of title need be obtained for:
.....snip...(10) Trailers having gross weight of three thousand pounds or less
Side note:
I think it would be illegal to represent a trailer as a camper in any State, and would be most uncomfortable making a purchase from someone who suggested it, perhaps having done it themselves?
EVERY trailer of reasonably recent vintage comes from its maker with what's called a "Certificate of Origin". That's the document used in all States/cases for original Titling. Hopefully you're dealing with the original owner of this unit, and that he still has that document or can acquire a copy. I strongly advise that you insist on that paperwork since you may eventually need it if, say you move or sell the trailer out of State where such Titling is required.
" Not every mind that wanders is lost. " With apologies to J.R.R. Tolkien