Ebay is one answer, although it is moot whether it is the obvious one. Some of the other hi-fi mags offer free reader classifieds and you could do worse than go down that route too. Benefits are you have no upfront cost - you will pay a hefty sum for Ebay's double dipping charging structure (if you choose to accept PayPal payments and with the recent change in practice, you now have no choice on that score) - and one month of free advertising as opposed to a maximum of 10 days auction. Admittedly, yo may already have an Ebay shop, where you have 30 days, but then you have the cost of the shop itself.
I'd weigh up the pros and cons of online auctions and classifieds in the hi-fi mags. Ebay is notorious for driving down, as well as up, the prices of various products - perhaps you could check the winning prices of completed listing for your speaker and likewise the secondhand dealer prices (there's a couple of used hifi sites you can use in this case). This will at least let you compare and come to a decision.
Good luck with whichever option you pursue!