The problem with old servers is that they are power hungry and noisy. Any software that needs/can take advantage of multiple CPUs will run on newer and more efficient hardware, anything that only needs a single CPU will run just as well on a smaller, quieter desktop machine.
Outside of owning for the curiosity or collection, I don’t think there will be much demand. If the CPUs are top end stuff then they’ll be worth more on their own than the servers as a whole most likely.