If it's in good shape I'd probably try it in the $1250 range. A little bit lower if it's a manual start tiller version. I just did a quick check on Craigslist here and I found 25hp's in the early 00's between $800 and $1700. However the $1700 motor is from a dealer in Gloucester that is always about $400-$500 high.
The key to getting good money for it is to advertise with clear details, good and bad, and use lots of pictures. Video of it running helps even more.
$1700 - https://boston.craigslist.org/nos/bod/4673707371.html
$1000 - https://boston.craigslist.org/nos/bod/4683057292.html
$800 - https://worcester.craigslist.org/boa/4606571190.html
$500 - https://boston.craigslist.org/gbs/boa/4685000985.html (slightly older, rough visually)
Good luck!