Tarot cards are one of the diviners most personal tools. Sure it is okay to buy them as novelty, especially if they once belonged to a famed psychic. But, i find that it is better to own your own deck. When using tarot cards it is important to tune one's clairvoyant vibrations to that of the deck, thus using the cards as a medium of strengthening and channeling predictive abilities.
When a diviner has used a set of decks for awhile they will begin to notice a sort of "personality" (if i may say so for want of a better word) that the deck adopts and soon find that they make predictions more accurately with that particular deck than compared to other decks. Think of it like when using a bicycle. After using once bicycle for so long, it is often "wierd" to be using another bike, even if its a new one. When we touch things, or use things, we impart upon them psychic imprints of ourselves, minute and mostly insignificant. Think the same with tarot cards, except on a larger scale.
Sometimes when using another diviner's tarot cards (especially if the diviner is not around to help your vibrations flow into his/her vibrations) one can find that the tarot card's predictions may be off (innacurate or unrelated to the question being asked). However, like a used bicycle one should be able to tune into the deck's vibrations with time and use it to almost, if not perfect effect.
An alternate viewpoint:
With the greatest respect to the above answer. Others believe that the cards themselves neither bear the imprint of the user, or maintain psychic energy. Ultimately this answer goes to the core of how one views the Tarot: (and I am certainly not suggesting an answer) Is the tarot a mystical source of divine insight, or is it a deeply introspective tool for self analysis and therapy? If you feel that the latter case better describes your approach to Tarot, (ie: the energy is 'you') then you should certainly feel comfortable with used cards. (Or online cards for that matter, like those at iFate.com)
The ultimate answer of course is a question: What do you feel comfortable with? What works better for you? Tarot isn't a religion. There is no dogma. Only practice.