"Silent traders", "shy guys", "unseen ones"; as many appellations for a species that seem to have only ever interacted with other civilisation of Merat through automated vessels. The planets they occupy are unknown for the time being, as their AI-manned ships don't communicate their whereabout and take great care to never been followed, doing great detours, killing their pursuers or self-destructing if necessary. They don't seem to originate from the Sentros Galaxy, which has been thoroughly explored by its various denizens.
Their vessels are exclusively dedicated to trading through a system of silent barter: products on display are only made available when an item of equal or superior value is placed in a dedicated spot nearby. Other meaningfull stimuli (like the alarms singaling the end of a bargain session) are taught in the chambers at the entrance of the ships in what amount to enormous Skinner boxes. Any communication is made through simple signals in multisensory media (with major components in visual and osmic spectra), ensuring almost any intelligent race can understand the functionment of the vessels, if only by trial and error.
Curiously, most vessels have unique but homologous designs, as if they were the result of various communities of builders basing themselves on the same initial concept.
While the secrecy with which the creators of the ships operate doesn't inspire trust, the amount of accidents while trading is very low and can always be traced to an attempt at stealing goods or at following the trajectory of the suspicious vessels as they launch. As a consequence, the "unseen ones" acquire more and more customers, while no one really knows about them.
This situation has fostered speculations and attempts to extrapolate from what little data the vessels themselves present. Many different analysis exist, often centering around the notion that the vessels may be all that remain of an extinct race, or even that the vessels form their own spatial civilisation.