Well, one of the MWDA books (Patriot's Stand?) has them go to Galatea where they are trying to hire help (Seven Samurai? Magnificent Seven?) and they walk into a Arms Dealer's showroom (think biggest & gaudiest car lot) . . . then out the back door to the used & junk lot. So yes, they exist . . .
I use the loot-type rules from WEG's Star Wars Pirates & Privateers for selling off things taken with the Alliance or a fence- or something similar as I remember them. Which means black market your are making out like a king if you get 40% of the value, gray might be 60% (yeah, you can sell those weapons to another approved entity . . . but you are ducking the tax forms, approved end user forms, etc) and selling to the faction/business that hired you might be 80%. Of course, I also think that buying a new or factory refurbished mech should be something like 130% to 140% of the listed value or if buying from a noble/other merc/security business maybe 110%-120%.