Sometimes equipment rental places will buy up lawn equipment to add to their inventory, especially if it's in good shape, and especially if it's an offer they can't refuse.
A combination of different media would be great. Advertise in online classifieds. Craigs list is huge, or so I hear. Put up a signage on your hard. Email friends. Announce it on Facebook and Twitter. Distribute leaflets on people's mail boxes. Tell everyone you know and let the word out via word-of-mouth.
At work we have an employees only web site. The site is there mostly for getting info on benefits and policy and for job postings. As kind of a "perk", they made a spot on site for classified ads. I have sold several items that way.
Hello Andyman! Why do you sell your used mowers? You have no use for them anymore? My friends use their mower until they buy new one or the mower is no longer useful. That's why I'm curious.
To me, a mower is just like a car. I'll continue to fix it my self until it develops problems that I can't (or don't want to) deal with. (Like fuel leaks.) At that point I'll get a new one and sell the old one to someone who knows how to deal with that problem.