Your Artisan Level and Gold are shared between all players.
So (let's say) you beat the game with your L30 Barbarian. He hops straight into Nightmare with all his skills, gear, etc.
But when you start a new L1 Demon Hunter, he's on Normal ... but with all the gold and Artisan training of your previous game. So you can kit yourself out IMMEDIATELY with some awesome gear and blow through Normal like a BOSS.
Regarding "how do I know it's good"?
- If it's blue, it could be useful for you, but isn't worth selling/trading.
- If it's yellow, it's probably good if it has:
+Int OR Str OR Dex (useful to individual classes)
+Vit (ALL classes need Vit)
+Other stats, especially regen or life leech
- If it's a legendary or set piece, it's BEYOND valuable. Hang onto it until the real-money auction house launches.