Checkpoints always existed (without any in-game purpose I'm aware of) in any System with an Active Conflict zone.
If by Merchants you mean "Seeking Weapons", those will appear in any System with Status "War" (not Civil war).
Not sure what precisely they buy or not, but I've read Reactive Armour, Non-lethal Weapons, Battle Weapons and Personal Weapons will all do.
So it seems the entire range of legal and illegal Weapons can be delivered to them.
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For selling Cargo, all you have to do is to fly within ~400m range of a locked Type-9 smuggler for a few seconds... and (I think) you now have to have your Cargo hatch open to agree to the sale.
Then, the Cargo is magically beamed in unlimited quantity (your entire hold of matching stuff) and you get a Message 'Received x.xxx.xxx Credits'
If you're unsure about what Commodity is specifically needed (some USS-Type9 smugglers are very cryptic) - you have to run a Cargo-Scan on them. Their Cargo hold will reveal what exactly they are buying.