The following is taken from the preface of Selling Middle Market Businesses: Guide Book for intermediaries by Russell Robb.

There is a movement afoot whereby business brokers are moving up-market from selling Main Street businesses to selling companies in the lower-end of the middle market, previously the exclusive domain of the M&A intermediary.  There is no precise definition of lower-end of the middle market as some people would clarify companies with sales between $5 to $100 million, while others would classify this category as companies with sales between $3 to $50 million.
 
There is a natural tendency for business brokers selling Main Street companies to move up market where the commissions are more lucrative.  It is as natural as real estate brokers who often start-off selling residential houses for $300-500 thousand to move up-market to eventually sell multi-million dollar homes.  But, the game is played differently at these two levels, and the valuation methodologies are dramatically different between Main Street businesses and the middle market businesses.
 
Business Broker vs. M&A Intermediary
 
In this preface, I will explain the major differences between the business broker and the M&A intermediary including the preparation and the process. The following is a short list of highly abbreviated differences between a broker and an M&A intermediary.

Characteristics Broker Intermediary
Size of Business Sales $100k - 3M $3M-100M
Type of Businesses Retail Stores Manufacturers
  Restaurants Distributors
  Service Service
Documentation Minimal Extensive Offering
  Brief Write-up Memorandum (30-50 pages)
Type of Buyers Mostly Individuals Strategic
  Locals Private Equity Groups
    National/International
Compensation Usually no Retainer Retainer
  10-12% Commission Structured Commission
    Minimum $150 to $500k
Sale Process Advertising Controlled Auction
  Email Blasts Specific Time-line
  Many Buyers Visit Seller Few Management Meetings
Firm's Advantage Number of Seller Listings Specific Industry Focus
Duration of Contract 12 months 6 months then Renewable Thereafter
Confidentiality Extremely Tight Involve CFO & probably Sales Manager
  After Hour Visits  
  Only Owner Aware  
Financials Use of very limited figures Extensive Analysis
  Past/future  
Decision Maker Founder/CEO Founder/CEO and/or Board of Directors
Fees Around 10% Commission Scaled depending on size such as:
         4% on $10M deals and 1% on $100M deals
         with minimums of $150k, $200k or $500k, etc.