Well, generally the seller's agent will try to use the comps to justify a higher price . . . at least as high as the listing price. On the other hand, a buyer's agent will try to use the comps to justify a lower price.
That's more difficult to do if there are a limited number of comps, or if the comps are really good matches. For example, in a large condominium, you may have condos identical to the one being sold. Maybe they're on the same floor, maybe on different floors. The floors make a difference in price, as does the condition of the condo. But--for instance--if you have comps of $215,000, $210,000, and $205 for identical units, neither agent is going to have a whole lot of wiggle-room. Sure: A few thousand more for being on a higher floor, and a few thousand more because the unit's in better condition. But the age is the same. The location (address) is the same. And so on.
There's a lot more wiggle-room if the comps aren't perfect matches. And there's even more wiggle-room if there are enough properties that the listing agent and the buyer's agent can actually pick different properties. Example: My sister and I recently sold my mom's townhouse. There were no comps in the actual community. Close by, there were some newer but smaller condo townhouses. A bit farther away, there were some about the same age and about the same size. You really could have looked at either grouping to have tried to come up with a good market value.
Nevertheless, a good listing agent will try his/her best to figure out what the market value of the property is. It doesn't help him/her or the sellers to substantially overprice it. That'll result in no offers. Similarly, a good buyer's agent--while negotiating for the best price--knows that if the buyers' offer is too low (substantially below market value), it's more likely to be rejected or countered. So the buyer's agent probably will suggest an offer at the lower end of what he/she considers a reasonable market value.
Hope that helps.