Loot everything - Every single item in the game has some value and you should take advantage of this fact. Grey items can be vendored for that little bit of extra pocket money and white stackable items are needed for crafting, which means that people will generally buy them if you post them on AH for low enough price (if you can't be bothered to post Auctions, you can just vendor all of them and be done with it). Some of the looted items will rise in value over time, so you might also want to take your time and store them on Bank Alt.
WoW Classic seeks to recreate the "vanilla WoW" experience—that is, WoW as it existed before a series of seven game-altering major expansion packs from 2007's The Burning Crusade to 2018's Battle for Azeroth. To achieve this, Blizzard has rebuilt the game based on archived data from back in 2005 and 2006 (patch 1.12 is the goalpost—the current game is on patch 8.1.5). The company has committed to meticulously presenting the experience exactly as it was back then—warts and all—with only a small number of unavoidable or critical changes.