I see lots of comments on here about cars disappearing after people spawn them. This has been asked and solved MANY time on the Zomberry Discord. You guys should really go there for help not here. Anyways here's the fix, worked perfect for my server.

need to set their lifetime to something larger than the default in types.xml. for whatever reason when the server spawns them they last, but they need a lifetime when spawned via admin tools or trader
<lifetime>3888000</lifetime>

Set that for EACH car and car color, change from 3 to that and your golden.