The Marketplace is supposed to take a 15% cut of the sale price. If the familiar sold for 40,000 plat, the 15% cut should have been 6,000 plat, which should have meant you receiving 34,000 plat.
Can you post a screenshot of your Sales history?
At the Marketplace, on the Sales tab, click the little gavel at the top-right (just under the red cross)…
This will show your Sales history, including the amount of plat you received for the familiar.
You’ll be needing this…