That's exactly my concern with the non-geared columns. The benefit of a Craigslist purchase is the ability to try them out. Bring along a pair of binos and ask them. You can also test out the height.
I just got back from testing out the Bogen 3021 from Craigslist-- you all called it right, a geared shaft is definitely needed. I could get fairly sensitive raising of the sliding shaft, but the shaft would rotate a good amount while I was raising it, which seemed problematic. I couldn't feel good about committing to using that tripod for the foreseeable future. For the record, I have 15x70s, so not necessarily light binoculars, but not incredibly heavy either.
I'll keep an eye on Craigslist for a good geared tripod, but I'm not too hopeful, as I'm in a smaller area. Anyone have any brands/models I should keep an eye out for?
At the end of the day, I'm thinking I'll go with Jon Isaacs' suggestion, and build a p-gram mount on top of my Benro tripod. I've of course got some questions about how to go about doing this, but I'll start a new thread for that, just to keep things concise. That thread can be found here.
Thanks everyone for the advice, it's very much so appreciated!