The main point of everything I've said is: I don't think there's any way to devise non-arbitrary rules whereby a player can't get ahold of a black market Crimson Dynamo suit if, during role-play, his character fairly manages to do exactly that. So if the GM wants to encourage Marvel Superhero style role-play instead of D&D-style teasure hunting, the discouragement for keeping that kind of gear has to come from events that are role-played out within the game.
Those are the choices, as I see it: arbitrary fiat vs role-play.
Beyond that, there's GM style. Personally, I think encouraging players to over-reach leads to interesting and fun role-play. But if you don't, and your player's still over-reach, I don't see any non-arbitrary way to rule-prohibit it. You're kind of left with saying, "Guys, superheroes aren't treaure hunters. If you keep the Dynamo armor and the X-Ray Lasergun, you're really not Mr. Street and the Green Roach anymore, that's not their concept."
There's nothing wrong with that. Find out what kind of game the players want. Why would the street-level superhero Green Roach want an X-Ray Lasergun beyond a specific, immediate need?
... yeah... I'm going to go ahead and pass on the opportunity to encourage players to act like 3rd grade powergaming uber-munchkins and get 80 stones worth of power for the 6 stones they invested in wealth.
Except that they don't
really get the eighty stones for six. They just believe they have. The price is paid not in terms of
stones, but
in terms of what happens in game events. The eighty extra stones they get from keeping the gear of defeated supervillains or buying stuff on the black market doesn't truly belong to them.
If Mr. Street beats up the Wrecker with his black market Crimson Dynamo suit and then trashes it, everything is fine. He couldn't beat the Wrecker with Close Combat, so he used Wealth and Black Ops to find a way. Great.
The trouble starts afterwards. The Wrecker has been beaten, no reason for the player to not go back to simply being Mr. Street, except greed for power. At that point, if he keeps it, it's like a hot potato. In effect, the PC acquires a number of challenges until he gets rid of the suit. The KGB wants it back, SHIELD wants to confiscate it, Justin Hammer wants to mass produce it, etcetera. Now the PC has to role-play all these challenges he didn't ask for until he ditches the thing.
In other words, it only
appears that the players are getting stuff for free. Run a few adventures like this, and the next time the players have a chance to buy black market light sabers, instead of grabbing them up, they'll arrest the crooks and either destroy or turn over to SHIELD any advanced gear.
Two answers:
One: the point of comparing Strength and Wealth is that what players invest their stones in tells you what kind of adventures they want. The player with Strength, Flight, and Force Blasts is expecting the chance to use these abilities in the adventue, probably in terms of big, public fights with supervillains. Similarly, a character who invests in Black Ops, Shapeshifting, and Targeting will want to use these powers, probably in very different ways. Wealth is a kind of power, and a player who buys a high Wealth ranking is telling you he wants adventures where its useful, perhaps even as a tool to help defeat the villain.
Two: why should Wealth give you a Crimson Dynamo suit? Because, frankly, it can. If that Crimson Dynamo suit is there on the black market, and a PC has both the Wealth and the Social Skills/Black Ops to get to it, there's no logical reason from
within the game universe why he shouldn't be able to buy it. Things like this happen in comics all the time. There is a black market, and this kind of stuff can be purchased.
As GM, how do you deal with it?
One way is by GM fiat: if you don't pay stones or lines of experience for it, you can't have it. I find that to be very fourth-wall breaking. Campaigns like this I leave as soon as I can find a better one. It's the GM saying to the player, no, you can't because you didn't buy it with stones.
Except from the character's point of view, inside the game universe, there's no such thing as stones. No logical reason within the game universe why the character can't get ahold of the Dynamo suit.
The other way is: events in the game universe happen so that things naturally balance out. The character starts buying up stuff on the black market, and instead of the GM telling the player he can't do that for a fourth-wall breaking reason, an NPC tells the character that if he doesn't stop, trouble is up ahead:
For example, Nick Fury stops by and says, "I know you've been scooping up ex-Soviet military ordinance. I'm not the only one who knows. Turn the stuff in now and I'll say you were working for SHIELD all along to get this gear off the black market. Otherwise, the shit is gonna hit the fan, and I'm not going to be the only one throwing it."
Now you've got a scenario going. Can't blame the GM; Nick Fury is well within his rights as a lawman to lean on the character. In fact, he's done exactly that in the comics numerous times. If the player keeps the gear, he essentially gets a number of off-the-sheet challenges:
The Red Skull wants the Crimson Dynamo suit. SHIELD tries to arrest him for owning illegal military ordinance. Vladamir Putin wants the suit given back to Russia and makes public statements to the effect that the character is a criminal and "this will not stand". And none of this is arbitrary because it all flows logically from what the character himself has done.
Trust me: if you do this correctly, you won't have to enforce book-rules about game balance upon your players. It'll get to the point where, upon getting ahold of an entire HYDRA arnsenal, they're more than happy to turn it in to SHIELD as oppossed to treasure-hunting.
And if they don't? If they keep going after every bit of power they can snatch up? In that case, they don't really want to be superheroes, they want to be supervillains, or vigilantes who are laws-unto-themselves. They ultimately end up as hunted men with both villains, law enforcement, and other superheroes after them.
If that's what players want, no amount of rule-enforcing will make them into the Avengers anyway. Ask them what they think they're doing and what kind of game they want.