I’ll just point out that when it comes to specific sales on z&g that there could be shenanigans even if something gets paid for.
$13k sale for Mewtwo 
Previously sold puts it closer to ~5k. Instantly after the 13k auction, before it was confirmed paid, I got a 7.5k offer on a copy in my Pwcc vault.
That’s an instant 2.5k increase based on the sale. Z&G charged 5% fees for that sale. You can also think of that as the cost of setting whatever pricepoint you want.
Let’s say I had 2 PSA 10 Mewtwo . I could list one on z&g, pump it up to 13k on an alt account - but actually pay for it. All I need is two or three ebay accounts, two addresses and $13k liquid to do this. But ultimately it only comes out to $650 cost because I am paying myself. Then, once I’ve established the new price and I get that 7.5k offer on fanatics, I’ve basically created $1850 out of nothing.
Not saying this is happening… but the incentive is totally there, especially if you create a new record price for a card you have multiple copies of. It’s also not the fault of z&g in this situation. They are just the means of implementing this method because the fees are low enough to make sense.