What to look for on sick crayfish.
The crayfish on the left is healthy. The flesh is a slightly opaque.
The crayfish on the right is ill. It's flesh is white.
All crayfish with white under the tail should be removed from others. Consumption of the flesh of a dead, diseased crayfish can spread the disease to the others.
A quarantine tank can be simple as a storage bin with water just over the crayfish's head. No special care is needed, as the crayfish will not drown in shallow water, and the crayfish will eventually die anyways.
This precaution will prevent spread of the disease until the crayfish can be returned to the vender.