Price seems reasonable. Compare it to this one on Gunbroker.
Marlin 1894 Cowboy Limited 45 Colt W/ Original Box : Lever Action Rifles at GunBroker.com
The JM stamp means it was made by Marlin before Marlin sold out to Remington. Remingtons are stamped REP. FYI, JM is stamped on the port side of the barrel near the receiver; REP is on the starboard side. Quality of the REP models is very poor. The JM stamp means it is a higher quality.