i wouldn't. Id be afraid that any "proper" sterilization effort (involving lots of bleach or boiling water) would have an adverse affect on the Bathmate's adhesives.
I don't see the problem myself. So long as its cleaned in a strong solution all is well. I've purchased 2nd hand extenders in the past with no problems. Just make sure you clean it with something medical grade and that wont damage the plastic i.e. its too strong and than if someone purchases it than they should also cleanse it before use. Simple.
It's not so much a hygiene issue, it's a phsycologocal issue. Your Bathmate is like your second dick. I remember losing my Bathmate virginity It was in july 2010, the 19th i think