I don't know about pokemen specifically, but for things like this in general my rule is if I can make a good profit on them sealed then I would sell them sealed rather than take the risk of opening them and not having any desirable ones and them staying on ebay at 99p each for ages. Logistically its easier to seell them in one go too. One listing and one trip to the post office and done.
You could take the chance and open them up, you might have one or two rarer ones that will more than cover the cost of the whole pack, but be prepared to have lots that have very little value on their own.