If both are in great condition, as you say, there is a huge market for such machines. That said, the prices netted will be what the market can bear, and often that is substantially below original purchase price. Go to Sears.com and price a comparable new one, sale price. Cut that in half.
If the new price is $1600, then ask $800. That is pretty much how it works. You might do better in your particular area, but lawn season is coming to end soon in the northern climes. Spring is always the best time, mid-May when the grass is as high as an elephant's eye.
Craigslist, I assume, or similar.