There is a large Brocante at Coutances called ” La Cle des Temps”.
http://www.lacledestemps.fr – 02 33 07 41 03
Although they don’t specifically trade in pianos, they often have one or two in there. They won’t buy it directly from you, but will endeavour to sell it for you for a fixed percentage commission. You’ll see from their website that they’re well organised and, because the site is well viewed, actually shift stuff.
You set the value – this would be important as transport costs might outweigh the piano’s value – Monsieur Lechevallier in St Lo is extremely helpful – http://www.pianoslechevallier.com – maybe if you contact him and send some photos he might be able to help.
Good luck. It’s always sad to see a nice piano go to waste – unfortunately many do.