So, this is the first Pokemon game where I've been able to do online trading, and I've learned one thing: Petilil and Vullaby are in very high demand. Both of these pokemon are exclusive to Moon, and pretty easy to catch (Petilil is especially plentiful). But, for some reason people (I'm assuming people with Sun) will trade their left arm for one. I farmed them and ended up getting all the sun exclusive pokemon, including the two Sun exclusive Ultra Beasts and even the cover pokemon, Solgaleo. Now, to be fair, I don't care about IVs or natures, a lot of this is just for filling the pokedex, but if you're more into collecting/using in-game then petilil's seem to be solid gold.
I've also used them to get a bunch of the hard to catch pokemon that technically exist in Moon (like Kangaskahn and that cool anchor pokemon) among others. I also got the other two starters, which I used for a bit but then bred them so I could have ones that obey me, so now half my team is the three starter pokemon (along with a Mudbray and Alolan Grimer I intend to keep, and a magnemite I like but might switch if I find a better electric type). I don't know how long the Petlil/Vullaby rush will go, I think having the Pokebank available in January might slow the rush down at the very least, but as of right now I can get almost any available pokemon in Sun/Moon by getting a bunch of very easily caught petilil then trading them.
I mean, I can't get the Moon exclusives that easily since no one with moon needs a petilil, but I'll get those on my own. Plus, I'm not going to spend time trying to trade for pokemon I can easily get myself. But, its been an awesomely bizarre time getting ridiculous trades for a few common pokemon.