Mycelium's Marketplace is meant for one-on-one transactions where you sit next to the trading partner.

A few hints:

• Arrange a meeting in a restaurant and sit somewhere in the back, not next to the door, to reduce the risk of a scammer running away. To increase your security, ask a friend to act as your bodyguard. Tell him to follow you in a short distance, so that nobody can see that you are together. Let him sit somewhere between you and the exit door.
• If you have cash in your pocket and want to buy bitcoins, do not describe yourself so the other person can find you. The person selling bitcoins should describe themselves, because they are not as much in danger of getting robbed at this time.
• Do not carry more than one or two months' of your salary in cash. I would say, no more than $10,000 for a middle-class person.
• If you carry a large amount of cash and something goes wrong with the communication, consider the risk of getting robbed as you walk away. For example, call a taxi and ask the taxi driver by phone to come in and escort you out.
• If you buy bitcoins, wait for at least one block confirmation and at the very least for 95% transaction propagation. Mycelium shows the latter as a bar graph in the Marketplace. It shows the block confirmations on the transactions page. Familiarize yourself with these features of Mycelium before you trade in the street. Ideally do a mock trade for a near-zero amount with a friend who also has Mycelium.