Working at a big university, I fequently sell my rescued/reconditioned/refurbished wonders by simply putting up a couple of flyers. When the kiddies arrive for classes in August, they all want a bike...
Likewise in the spring after graduation, when Summer-school and graduate students take up residence.

I've also had good luck using Craigslist, though you have to watch out for Nigerian scams... As if someone from London would want to buy a 150.00 hybrid...