It's called Rare monster hunt!
This is very simple, just download the following addons:
_NPCscan
_NPCscan.Overlay
This addon will pop an alert when a rare monster is nearby so you don't have to create a targeting macro and spam that. If you download the overlay too, all TBC, Wotlk and Cata rare spawn points will be shown on your map so you can easily see where a rare might be found. It's always worth it to check them when you're in the zone.
Rare loot is rare, so there is no point asking for less than 1000 gold for a piece of equipment unless someone is undercutting you. For example, I've been able to sell Armagedillo's Tail for 2500-3000 gold everytime, even though even Obsidium Executioner is better, yet a LOT cheaper! (Don't tell anyone!) :O
Note: Wotlk rare loot is harder to sell, so there is little point farming Northrend rares, except those in starting areas. Some TBC and Classic items however are very valuable!