They are either old or have been crushed if they are leaking. Look for dry packages. Avoid berries that aren't their prime ripe color. There should be no pale or white patches on the fruit. Blackberries should be deep blackish-purple.
Berries freeze well.
A great idea is to grow your own or pick some from a local farm. Then store them in your freezer for year round treats. Also, when juicing some berries, you will get more of a puree than a juice. Raspberries and blackberries can be run through the juicer several times to get the most from the pulp. Or you can juice the berries and then follow them with a thin juice such as apple to chase the berry puree out of the juicer.