The Robins signed Maynard from Crewe for a club-record £2.25million in 2008 and the 25-year-old has repaid that faith by scoring 45 goals in his 113 appearances for the club.
Talks on a new deal have been ongoing since the start of the season but with no agreement on the horizon, City have opted to protect their investment by putting him up for sale.
"For some time now Nicky has had a very good offer on the table, showing the club's ambition and our desire to secure his future with us beyond the end of this season," manager Derek McInnes told the club website.
"Nicky has made it clear he does not wish to enter into any more discussions at this time.
"As much as I would like to keep Nicky at the club beyond the end of this season, we are left in an unenviable situation.
"We must protect the club and, therefore, we have no alternative but to seek offers for Nicky during the forthcoming January transfer window."