I do understand your predicament - sorry if I answered in more general terms. In your situation specifically, I would still contact the buyer with a friendly welcome and then inform them that the assessment is due. Under the Disclosure laws, Title 33 section 173, subsection 6, the seller is obligated to make disclosure to the buyer as follows:
6. Access to the property. Information describing the means of accessing the property by:
A. A public way, as defined in Title 29‑A, section 101, subsection 59; and [PL 2017, c. 181, §3 (NEW).]
B. Any means other than a public way, in which case the seller shall disclose information about who is responsible for maintenance of the means of access, including any responsible road association, if known by the seller;
If the buyer participated in a sale without a realtor involved, unfortunately they took their chances on whether proper procedures were followed. As I said in my prior response, After a friendly greeting I would let the buyer know that regrettably the seller was not up to date on road association payments; that payment is now due, and that in order to have a clear title they need to pay the assessment, but that they should then seek reimbursement from the seller.
If they do not pay, I might offer them grace up to the full 60 days from when you contact them, (rather than from the original due date,) and then proceed with a Notice of Claim.