What measures you need to implement depend largely on whether you're going to work the gray market or the black market. The essential difference between them is whether or or not the goods or services you offer are proscribed. If what you're up to would be legal if you submitted to the State for permission, it's the gray market. If you're peddling something illegal -- guns in some areas, banned agricultural products, paraphernalia that's legal if used with certain drugs and banned if used with others, and so on... --- that's the black market.

Gray Market
Some gray market activities are an American tradition: baby-sitting, mowing lawns. People think nothing of it. Others, like roadside fruits stands , are about as accepted, but occasionally the cops want to see a peddler's ticket. Other things are guaranteed to get you noticed.

• Advertising should not be the sort that garners government attention. Word of mouth is best, but bulletin board notices can work well. Be careful of classified ads, since some cities actually have people to watch them for recurring ads that indicate an individual is conducting a regular business without a license. For instance, some locales forbid yards sales more often than quarterly, unless you meekly buy a license from them.
• Give value for money. Don't give a an irate customer a reason to report you to the cops.
• If you run a roadside sale, whether a fruit and vegetable stand or consumer electronics from the back of your van, stay alert. If you see a police car cruising past more than once, pack up and quietly move on.
• If the cops show up, don't argue, or otherwise irritate them. Too many officers these days are looking for an excuse to bust you, or at least your head.
• If you are going to be selling guns at all regularly, don't sell them at gun shows. Feds do cruise shows looking for familiar faces making "private" sales.
• Flea markets are good places to set up shop. In theory, you're operating under the market operator's business license, but rarely does anyone try to track your earnings. Rent your stall space with cash, and give a fake name. Keep using that name in your stall.

The gray market is really just common sense.

Black Market
This is a whole different ball game. Because you're selling illegal goods, you need to do more than simply avoid official notice. In principle, the cops are actively seeking you out, if not by name.

Also, because banning products effectively acts as an artificial price support, gun or drug running can be very profitable. Which means that existing dealers may not appreciate you moving onto their turf. But muggers might.

• As mentioned, black marketers don't always welcome competition. Before you move into an area in a big, do some market research to see if the community is already served: put the word on the street that you want to buy whatever you happen to plan to deal. If you get a lot of offers, you should probably try another area.
• Work with a partner.
  If you are retailing to individuals, you may want your partner to be an invisible scout and backup. He eyes an area before you come in with the incriminating goodies, and watches your back during deals. He should be covert so no knows to scam him or take him out.
  If you're wholesaling -- a crate of handguns, perhaps -- you want at least two partners. The first scouts a meet location and maintains surveillance on it for a minimum of an hour before the meet. He lets you know if it's clear, and guards you from the figurative shadows. He must be covert.
  The second associate is overt; a visible bodyguard to assure your buyer that you aren't candy.
  Arrange duress codes for yourself and your bodyguard, so you can tip each other off without alerting your "buyer." Likewise, have some nonverbal signals for your covert scout. Wouldn't it be nice to have some rifle backup to settle sudden conflicts?
  
• Don't meet with large groups; individual representatives are safer. If your scout advises that more people have shown up than are really needed to move and protect their purchase, you shouldn't make an appearance.
• You always set the meeting time and place. If the other party picks, they can choose a place where they have pre-positioned backups to ambush you, whether they're cops looking for a bust or buyers looking for deep discounts.
• Pick your meets carefully:
  • It should provide your scout with a good hiding place with a view of the meet.
  • It should have at least two escape routes.
• Don't reuse meeting locations. Don't set regular days or times. Don't be predictable to cops or to unethical people looking to relieve you of excess profits.
• If the deal feels sour, assume it is. Get out. If your buyer is late, don't wait around for him.
• If using a vehicle, don't use your own. Consider renting a car and temporarily replacing the license tags with a set you "borrowed" from some stranger's vehicle.
• If some ill-principled person might misuse your merchandise to commit a crime, don't handle it in a way that leaves fingerprints. It would be a real shame if the cops lifted your prints from a Raven used to rob a convenience store.
• If you sell the ammunition for that gun, don't leave prints on the cartridges either. Unprofessional criminals tend to leave autoloader-ejected brass at crime scenes.
• Never ever sell a loaded firearm. Some clowns out to make a name have been known to give in to unfortunate impulses.
• "Advertising" should be word of mouth, with sales only to those who've vouched for by someone in whom you're willing to vest some degree of trust.
• You may find it useful to create a business "persona" for your illicit transactions, which has nothing in common with your above-board-law-abiding-good-citizen true name.
• Consider buying a collection of of pre-paid throw-away cell-phones for your business contacts. Use one for a few days, then abandon it on the streets somewhere (carefully sanitized of any identifying tracks like fingerprints or programmed numbers in the phone book). Never use those phones to call a number tied to your real world identity.