There are two things the buyer can do to ensure compliant fuel oil: Firstly, the contractual agreement must be clear in terms of the specs, the sampling, the obligations of charterer & owner, and the determination of procedure in case of a dispute. The other thing is to ensure that the parties follow the agreements, primarily to ensure that the sampling (which usually is the contractually binding sampling procedure in the contract) is witnessed and ensured to be exactly as per MARPOL procedure. Note that the MARPOL procedure is clear in terms of method, but leaves the sampling point to be agreed in the contract. In most contracts, bunker suppliers offer basis MARPOL sampling procedure, with sampling point at Barge Outbound Manifold.
It is not a secret, that there is often a difference between the specs and terms the shipowner wants, and those a supplier is able to offer, and attention should be paid to such differences; most suppliers are making a very good effort to accommodate the requests from their clients, but often they cannot comply fully, as their cargo specs or supply procedures may differ and they will usually offer based on what they are able to guarantee.