You could ask the seller why he wants to part with them. If the reason is suspicious, or you get the feeling that the seller does not sound trustworthy, maybe you should walk away.
If the wheels are off the bike, and preferably also tireless, examine them real close under a very bright light. Any irregularities might become visible. Thick Carbon Fibre doesn't bend, it cracks. Should be recognizable under a very bright light.
If the wheels have tires on them, you could do a downpayment under agreement that you take the wheels home for a few days to see if there is anything wrong with them. Get the tirepressures to the recommended value and see if they hold that pressure. If all turns out well, you pay the remaining sum. If not, return the wheels.
If the seller doesn't agree to those terms, that could be another reason to walk away, unless you are very confident after inspection at the seller's.