The current method is to go the page: support.fab.com/s/?ProductOrigin=FabSupport
and select the Refund category, Make sure to include all important detail about the your account detail that you used to login, transaction, product url etc..
Note: You can only contact fab via that support page.
And to spot potential bad sellers, if the description sound like ai generated text, better not take the risk of buying it (especially when the seller set the generated with ai to No)
For a pack of assets, i would expect a demo downloadable file and some blueprint file if they were drivable.
And if a seller did that much effort to make so much cars by his own, there no reason he wouldnt include unreal engine / unity.