Look at eBay, but be sure to check SOLD items, not just open "Buy it Now" listings. Some of those folks are enjoying their keyboard while waiting for the right sucker, I mean buyer, to come along!
Also- you have to baseline your price against the condition of the others that have sold. Was your keyboard gigged out in smoky bars, or kept in a pet-free studio? Do you have the original box and packing materials, music book stand, etc? If you've never sold anything on eBay before, some bidders will view your auction with suspicion or expect a discount compared to sellers with an established reputation.
Along with eBay another site I browse often is reverb.com. The same considerations apply. Good luck with your sale!