In 2285, the Merchantman was a small freighter which carried classified Federation data regarding the Genesis Device to a Klingon Bird-of-Prey that was under the command of Kruge. The Merchantman was destroyed when Kruge learned that Valkris, a Klingon operative traveling aboard the ship, had seen the information. (Star Trek III: The Search for Spock)
The stage directions in the revised final draft of the film's script suggested that the ship was in Organian space when it was attacked and destroyed. Also in the script, the craft's exterior was described as "a dark-looking merchantman" whereas its interior was characterized as "small and grubby." 
The interior of the Merchantman was represented with a set on Paramount Stage 5. (Information from call sheets)
For more about this model, see studio models.
According to Issue #143 of the Star Trek: The Official Starships Collection, the Merchantman was approximately 150 meters in length, had a maximum speed of warp factor 5, and had a crew complement of 3. This ship was equipped with phasers.
The Starships RPG sourcebook describes this type of ship as a Class III tanker.
In the RPG module The Serpent Factor, published in Stardate Vol 1 #1, the Merchantman was instead called the SS Eileen.