In Lagos, there is a Law to provide for the prohibition of Street Trading and Illegal Markets in Lagos State and for other matters connected therewith.
The Law is called the Street Trading and Illegal Market Law of Lagos State, 1984. 
Section 1 of the Law provides that no person shall sell or hawk or expose for sale any goods, wares, articles or things or offer services whether or not from a stationary position in any place or street specified in the First Schedule to this Law or within the vicinity of any public building in the State.
Section 4 also prohibits the use of pedestrian bridges as illegal markets. 
Section 11 states no person shall sell or hawk or expose for sale any goods, wares, articles or things or offer services whether or not from a stationary position in any place or street specified in the First Schedule to this Law or within the vicinity of any public building in the State.
The law also provides for the penalty for breaching the law, it states that any person who contravenes the provisions of this Law shall be guilty of an offence and shall be liable on conviction—
(a) as a first offender to a fine of N5,000 or to six months imprisonment with hard labour;
(b) as a second offender to a fine of N10,000 and nine months imprisonment with hard labour; and
(c) as a third offender to a fine of N15,000 and one year imprisonment with hard labour.
Whenever you are in Lagos and you see street traders, note that they may be breaking the law. 
@Legalnaija