I'm from GB, never heard of any of these! Hawker would be my choice, it's what I know them as, someone who hawkes his wares.

Visual thesaurus (www.visualthesaurus.com) defines "Marktschreier" simply as "crier": a peddler who shouts to advertise the goods he sells. So I'd use "crier" or "(market) crier".
hawker: Someone who travels about selling his wares (as on the streets or at carnivals)

The American Heritage® Dictionary of the English Language: Fourth Edition.
http://www.bartleby.com
Crier
NOUN:One that cries, especially: a. An official who announces the orders of a court of law. b. A town crier. c. A hawker.

@ Selkie: And what is wrong with LEO's suggestion:
'carnival barker <-> der Marktschreier (auf Jahrmärkten)' [or: Fischmärkten] ?

From WordNet 2.0 :
Crier : (noun)
1: a person who weeps [syn: weeper]
2: (formerly) an official who made public announcements [syn: town crier]
3: a peddler who shouts to advertises the goods he sells

Chambers 21st Century Dictionary
http://www.chambersharrap.co.uk/chambers/chre...
barker noun
1 a person outside a circus or show, etc who shouts to attract customers.
2 a person or animal that barks (see bark1).

Some more information about barker:

Oxford Advanced Learner's Dictionary
http://www.oup.com/
barker /bk{shwa}(r); NAmE brk/ noun
a person who stands outside a place where there is entertainment and shouts to people to go in

The American Heritage® Dictionary of the English Language:
http://www.bartleby.com
barker NOUN:
1. One, such as a dog, that makes a bark or a barking sound.
2. An employee who stands before the entrance to a show, as at a carnival, and solicits customers with a loud sales spiel.

Cambridge Advanced Learner's Dictionary
http://dictionary.cambridge.org/
barker
noun [C] OLD-FASHIONED
a person who advertises an activity at a public event by calling out to people who are walking past:
a fairground/circus barker

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Go with (market) crier. Barker doesn't really fit, and mountebank is more like a charlatan, as far as I know and is used in negative contexts. I couldn't find any English dictionary in which puffer is described as a Marktschreier. The only one I could find said a puffer is (amongst others) a by-bidder.

Oops, didn't see the date there... ;o)

@Fury (BE)
Hauptsache mal kein Spam ;-)