Record exactly what differs
Compare the complete numbers rather than only their last few characters. Write down which document or vehicle marking contains each version, without changing either one. A transcription error is one possible explanation, but it is not an explanation you should assume.
Repeat the reading carefully
Check that you are looking at the VIN rather than a part number or another identifier. Improve the lighting, look directly at the marking and ask for an uncropped photograph if you are checking remotely. Do not remove, alter or interfere with an identification plate to investigate it.
Ask a specific question
Ask the seller to explain the exact difference and provide the correct paperwork. If you are viewing remotely, request a continuous walk-around showing the car and its accessible VIN marking. A video can help you organise the question; it does not independently authenticate the vehicle.
Keep the purchase decision separate
Do not let an attractive specification or a reassuring verbal answer replace resolution of the mismatch. Pause the purchase until the vehicle identity and paperwork can be checked by an appropriately qualified professional or the relevant registration authority. Preserve the original messages and documents so that the unresolved issue remains clear. If a corrected document is supplied, check the full number again rather than assuming that every discrepancy has been fixed.