No, not all LEs increase in value. If you have an LE 5,000 it may have lost value. Value is subjective based mostly on how many people don't have it and want it. Normally the lower the LE size, the better the chances, but still no guarentee. The other major factor is the pin subject. Tinkerbell, Stitch, Haunted Mansion, Pirates of the Carribbean are better than say Happiest Millionaire, Fox and Hounds, Oliver and Co.
E-bay is sometimes a helpful barometer, but don't rely on it too heavily. Sometimes people bid excessive because they don't know what to do with their $$.
Pinpics may be a better gauge, but still not perfect. Take a look at the # traders/# who want. The closer to 0, the higher the pin value. My threshold level is 1. If the number is greater than one, then it's not a great pin to have in my trade pile and I'll use it for lanyard fodder. The problem with this method is that a number of people registered in pinpics do not keep their pin traders/wants up to date.
JMHO.