Rob do you mean buy as in a resale ( purchasing their points interest up until 2042) or "renting points" ( paying someone for the use of their points for ONE booking/year) ?
For a resale you would go through a proper resale agent and it should be done through a lawyer.
For renting , while some of the payments can be made as an "escrow" ( held by a neutral party until both sides agree the deal has been completed) you are trusting that the seller doesn't cancel the booking and not tell you. Personally I'd only rent to or from someone I know personally and that I feel happy with their "honour" , but it's still a risk from both ends JMHO