Many people still say that Tarot decks and Crystal Balls should only ever be given and not bought. This stems from the age old tradition of the gift of sight and arcane knowledge being handed to, or passed to another, rather than being taught or bought. The cards or crystal ball would then undergo a cleansing ritual and a bonding ritual so that they would work with the new keeper, who would then be able to hear and see the messages.
I have some wonderful second hand cards, some are very old and have been through many hands, in fact some haven't been printed for years so its the only way to get a set. Some I had to cleanse and some didn't need it.
If you are worried about previous owners then its best to settle your fears and perform a short exercise to clear them of any negative energy and let them bond with you. There are so many ways, some have already been mentioned, passing the cards through the light of a flame, smudging, sound, incense.
I think the easiest must be the 'wake-up' I perform on all my cards before working with them, it not only lets the cards know I am ready and would like to work with them, but it infuses them with my energy and as it does so all other energies leave. Its very simple, its in part one of the 2007 Mystic Board Tarot Course