For instance look for epic TBC gems that may be sold for less than a gold a piece (cut or uncut). These will sell right now for 50-100g, as they are comparable in stats to blue expansion gems, which may be hugely expensive due to scarcity.

Also, people tend to estimate (wrongly, in my opinion) that really rare world drops for characters in their mid 60′s or mid 50′s will drop in value almost to zero. Therefore, some long term investment can be made in gear that was once in the 1000g range and now can be found for 10-50g in “liquidation” sales. Even crafted lvl 70 epics are still valuable, as they are better than any green and most blue that drops from Northrend until around lvl 75.