
Apocalypse Arms Scourge Railgun
Lacking a better infrastructure for the sale of goods between private citizens, one common means of setting up a trade is the intrasystem communications network (also known as "Chat"), and another one I am aware of is the "Subreddit Discord Server". Once a trade has been set up and prices agreed upon, the safest way for both parties to transact is to make use of shared storage, officially known as "external storage". Using external storage makes it possible to safely transfer small arms between citizens while inside of an armistice zone, greatly reducing risk for both buyer and seller. Certain ships currently come with such storage compartments, such as the Crusader Hercules series or the Anvil C8R Pisces, to name a couple. Another common type of external storage is 1 SCU and 2 SCU crates, which must be procured by either party. Trading with an untrusted party outside of an armistice zone is not recommended for reasons that should be obvious. Even inside of an armistice zone, risk is never completely eliminated, and the buyer or seller could simply choose not to keep their end of the bargain. My limited experience tells me that this risk can be mitigated when trading multiple items by processing transfer and payment one item at a time. While there is no immediate recourse for transgressions, the name of the offender could be written down for eventual retribution, or, in the case of the aforementioned "Subreddit Discord Server", their reputation publicly tarnished.
For reference, the weapons that seem to be of worth as of 2953 are the Scourge Railgun, Animus Missile Launcher, GP-33 MOD Grenade Launcher, Demeco LMG, and all sniper rifles. More common weapons like the FS-9 LMG may also be of worth when trading in bulk.

Klaus & Werner Demeco LMG
In addition to small arms, certain rare armor parts may be of interest to some citizens, and you could do worse, if time and the situation allow, than to pick up rare-looking armor pieces and store them, just in case.