Twinks like all the enchantments obviously:
In the 19 Bracket:
FCs want Stamina + Armor
Most DPS want Attack Power + Crit (since this is in demand by raiders this is obviously the highest cost I'd expect.)
Healers like Healing Power
A few mages / locks might prefer Spell Power to survivability, but since they mostly bring CC to the table demand would be lower.
Early on prices will be very high:
I'd expect you could reliably find buyers sell at these prices, but others may have other ideas. If you find highly motivated buyers you might be able to get much more. Prices are assuming a high pop server such as Herod.
Attack Power: 5-10k
Stamina: 2-5k
Healing: 5k
Spell Power: 2-5k
All these prices are considered on the low end of what you'll hear people pay, but there are a fairly limited number of buyers so finding ones who are actually ready to pay is going to be trickier I'd expect (assuming you are trying to avoid real money transactions, and transactions with obviously purchased gold.)