I am new to pin trading myself and found myself baffled as well. As you mentioned, they aren’t really official pins. But as with all pins, people attach value to them and that makes them “rare.” So I think it all depends on the collector.
Alot of the collectors I have seen focus on Walt Disney Imagineering, Disney Studio Store Hollywood pins, and ACME pins. Pretty much any pin with a low LE number really. I can’t say there is an advantage but im sure there is a niche or group that does collect and trade them. It really all depends on your personal preference. In terms of trading, I will let others answer though I know for me I would prefer not to trade for fantasy pins.
LE of 15 is an insanely low number. Usually fantasy pins range in LEs of 50-200. Fantasy pins are great in that it allows fans to own pins that Disney never made and quite possibly will never make. This could be rare characters, but also, sadly, Rule 34 sometimes applies. Usually they get people on Etsy or Instagram to prepurchase designs, but sometimes if they don’t get enough orders, they will make them anyway because they want it for themselves, and can’t get just one made. Preowned, or creators desperate enough to unload their inventory, will use eBay. I know the characters I have saved searches for rarely will have fantasies being listed, despite the amount on Etsy.