I kind of figured back-alley deals and fences were going to be a staple in the game.
I think the mechanic in GTA IV was pretty good, though it could use some tweaking. Once you call your contact, you set up a time to meet them, rather than them just magically appearing nearby. Take too long or otherwise miss your target window, and they take off before you get there.
I do like the feel of the back-alley, out-of-the trunk deal, but I'd also like to see the chance of cops crusing by, and busting the deal, or some Boostergang that's feeling froggy, thinking they can come up on free warez / gear.
If they could cross that, and the black market stalls in the mall, and the stores of Sleeping dogs............... it would be awesome....
Like I said elsewhere, really each section of the city should have stores that offer items that fit that area....
The combat zone and nomad areas should sell leather clothes, football pads, armor made out of tires and street signs... their weapons should be crazy taped up half broken things and older tech guns like old school revolvers, pump shotguns, etc.. the vehicle mods should let you get rusted dented pain jobs and have barb wire, plate armor, cowcatchers and ram bars put on your cars....
The regular city areas give you normal clothes, normal body kit, engine upgrades, and colors for your vehicles, and normal modern (for cp 2077) handguns, rifles, etc...
The corp zone shoudl have stores that sell you high fashion, really good body armor, metal gear, armored clothing, edgerunner fashion with all the glowy bits and transparent portions, and whatnot... the vehicle options should be the same as in the city but with the added options of armoring your car without losing its style, maybe some james bond defensive gadgets.... weapons should be the high end goodness, like Malorian 3516's, New Assault rifles, mono katanas, etc...
Heck maybe some military installation can let you get your hands on Railguns and am-3 Rifles, Tsunami Ramjets, Hardsuit style Power Armor....