Ordeal in Otherwhere — Andre Norton

When an outlaw seized control of the colonial planet Demeter, Charis Nordholm decided that it could no longer be her home. So she contracted herself into the service of an off-world trader, who assigned her to buying cloth from the mysterious Wyverns.

At first she was attracted to these shimmering dream-rulers of Warlock, and yearned to learn their secrets. But by the time she finally began to understand the menace of Warlock, it was almost too late.