If you are buying stamps like this one, condition really matters. Look for one that is centered on all sides; does not have any missing perforations or tears; is clean; and if it is used try to pick one with a nice neat cancel - ideally showing the post office, date and time it was cancelled. If the stamp is a higher value make sure the cancellation is a postal one rather than a revenue cancel. All of these things contribute to the value of the stamp.
In the case of the stamp you showed, the watermark also matters. There are two watermarks of the Hong Kong Victoria 2c (Reddish) value. It is either a crown with CC under it, or a crown with CA under it. (The letters will be reversed if you view it from the back of the stamp.) See the examples below.
There are also some color shades which is why I put (reddish) in the listing. The colors range from Dull Rose to Rose-Lake and even Carmine. (Lighter to Deeper shades of red). In the case of your stamp the condition is such that it makes the other factors less meaningful. But it is a good reference item if you have other Hong Kong 2c values.
