For example, I work as a software developer for a real estate brokerage, and have mentioned the sell side realtor scenario you bring up.
The answer is always: yes, it is in the short term interest of the realtor to take the first offer. But in the long term (as they screw more and more people) they will be hurt by this behavior.
In fact, there is long term incentive (because, in the US at least, people buy and sell homes multiple times over a lifetime) to get the best deal for the seller, so that they will: * refer the realtor * use the realtor again
To bring it back to recruiting, if you had a relationship with the recruiter, and they knew you were going to go back to them in 1 or 5 years when you were ready to move on to your next job, they would have a rational interest in finding you the right job (and the same interests hold true if employers have a relationship with the recruiter).
This relationship, if it exists, really turns the recruiter into a headhunter. (Aside, here's my favorite headhunter/recruiting information source: http://asktheheadhunter.com/ )