Hi Guest,
Here is a link to a blog that contains links to sites that offer corresponding dates for serial numbers. I'm sure there are a couple that will help you find what you are looking for.
http://www.mypianofriends.com/links-for-piano-serial-
number/dates-99.html
Though some sites offer to find the value, I would recommend having a professional give it a visual inspection. It's the only way to get a proper value, since all factors are taken into account. (Maker, age, condition, etc.)
As for selling it, there are online classified sites, both free and for a fee, and if you enter "piano magazines" into your favorite search engine you will discover there are a variety of magazines and they might have more to offer in the way of advice for selling.
I do know, however, that some retailers offer consignment. If this is a preferable option, then contact your local piano retailers and see who has the best offer.
Best wishes,
~B~
