So while not having checked all the trading threads here nor at discord IMO cards most often are searched for are staple URs like Remnant LS, Chronicles of the Kings etc. or staple Rares like Endless Treasures. Then after new sets have been released people seem to look for often used Rares/URs in addition like Joshua, the Conqueror for example.
At least I've been also looking a very long time for old staples like e.g. Nebby, or old often used cards like Magic Charms etc. (luckily I finally was able to trade them after all). So this is what I've been observing.