What would a used Blackberry Pearl sell for on ebay?
It works perfectly, but has a partial water damage.
The water damage is only visible on the censor under the battery. It is a white square that turns red when it gets wet. One half of that is red. Otherwise, no visible damage, and everything on the phone works fine. I just decided to upgraded to the Curve.

A Pearl with a missing battery door latch recently was sold for $66 plus shipping – another one with the same problem for $91.
Perfect used ones, or ones with minor scuffs, are going for $112 – $153.
A lot depends on how visible the water damage is and how much it detracts from the appearance, but I’d guess you could get $60 for it.
Ensure you include good photos of the damage in the listing and give some background on how it got damaged — people are more willing to bid if they understand the provenance of the item.
Are you sure it can’t be repaired? http://www.gethightech.com have videos on how to take apart an 8100 and it’s possible you could clean up the unit to make it look better.