..on a set of carbon fairing/aluminum rim wheels that I'm looking at buying.
The seller says that it was lightly crashed quite a while back, and much more recently one spoke has come loose repeatedly. He says that he took it to a bike shop and they couldn't fix it for him (maybe they just wanted to sell him a new wheel, since that was their suggestion?) Though he said that they also didn't have any replacement spokes.
Seems to me that a new spoke & nipple would be the worst case possibility- is there anything else that could cause a particular spoke to come loose on a weekly basis?
FWIW, the wheels are these: http://www.real-design.com/2005/ultrasonic.aspx