eBay is probably the best outlet. Values are typically driven by the size and complexity of the tube, and of course knowing if the tube still works. It's not uncommon for these tubes to loose their vacuum, thus rendering them unusable. I haven't looked at the values of Geissler tubes in many years but I would try and verify the working condition of your tube and list it with a reserve of around $75 or so assuming it still produces a glow. Photographing the tube lit will surely help an auction description grab the attention of more bidders too. It's a good idea to also research past auction sales - just search eBay's completed auctions for the keyword "geissler" or "crookes" to get a feel of values.