I agree with the majority here- use the same agent and it will be easier, assuming you are buying and selling in the same area. It will help so much with the coordination, which can get tricky. The comment: "If you buy a house, which is listed by the same agency as your agent is working for, then it becomes dual agency" is technically true, but in practice it is resolved before it happens. The agent you select to represent you will become your "designated agent" in that situation and will represent only you.
As far as the skills go for listing vs selling, I personally think a good agent needs ALL the skills mentioned in a previous post. I have found that representing buyers has helped me be a better listing agent- I know what the buyers are looking for! And the listing agent sure needs to be able to do a great analysis and market valuation to help you price the house correctly from the start.
Anyway, good luck, and as others have said, choose someone you feel very comfortable with!