What an inspector actually does
An independent inspector draws samples and takes measurements at the loading port, the discharge port, or both. They certify what they found. They do not certify that the cargo is good, only that it measured as stated at the moment they measured it.
Who appoints matters
Whoever appoints the inspector pays them. Both parties usually accept a mutually agreed name — SGS, Intertek, Bureau Veritas, Saybolt — precisely so neither is choosing their own referee. An inspection by the seller's own staff is not an independent inspection and a buyer should not accept it as one.
Load or discharge governs
Quantity always differs a little between load and discharge — temperature, measurement tolerance, vessel experience factor. The contract must say which figure the payment is based on. "Load figures final" and "discharge figures final" allocate that difference to opposite parties, and both are common.
Sealed samples
Samples are drawn, sealed and retained by all parties. If quality is disputed later, the retained sample is the evidence. A transaction with no retained sample has no way to settle a quality argument except by negotiation.