A range of rifles and sidearms - all illegal. You can also buy other illegal items that are not sold at regular markets, such as blasting charges for the HM Treasury vault or Code breakers for the RBA.
All you need is a license that costs 500k. ATM is on site.
As Rambo mentioned you get the usual pistols you get some okay 5.56 rifles some 6.5 and some 7.62 rifles you can also buy stuff like rangefinders vests,masks,rebel/army helmets rebel clothing and some granades and such
The Black market is a safe way for you to buy your rebel gear and if available you would be under the protection / surveillance of UNMC members to prevent trouble.