Key words are very important when advertising a business for sale online. Many business buyers utilize key word searches when they are looking for a specific business to buy. I always recommend to my advertisers to put a "Key Words" section at the bottom of the long descripition of the ad copy. Include every type of word that relates reasonable to the business your are selling.
A great example of this is
Steve Fitzgerald of Acquisition Services Group in San Diego who revamped his BizBen ad today - a specialized
Underground Contracting, Excavation Service that needed more exposure. His copy in his ad was good but if any serious buyer was doing some keyword research his ad would probably be passed by.
The keywords he put in for this business included: backfilling, back filling, compaction, sewer, water, storm drains, pump stations, commercial projects, municipal projects, general contractor, contracting, underground, under ground, excavating
Bottomline - when placing a business for sale ad online pepper your copy with key words or place them at the end of the text like Steve did in his ad.