Avoiding Recruiting Agencies
I work for small companies as a contractor out of my house. 'Small' means businesses with 10 to 100 employees. In some situations I have someone who is an 'agent', he found me the work, he pays me, and he marks it up and passes it on. This individual is highly knowledgeable in the IT world. Many of my clients come straight out of Craigslist. However, I have A) about 40 years experience doing this, and B) I tend to take on work that would terrify a lot of developers: various kinds of reverse engineering, and work that involves rarely used databases, complex stored procedures, or users that can't articulate what they want.
I get approached all the time from 'body shop' recruiters, usually to work full time on site. I tell them I have commitments until the middle of next year (more or less accurately), check back then. They usually do, and I'm still busy with newer commitments. There are circumstances that I get ugly with recruiters, but probably anyone else would given the same provocation.
Using Recruiting Agencies
Some large companies do all their recruiting through agencies, or all recruiting of certain types through these agencies. This is particularly common if the company goes through periodic cash squeezes and has to discharge full timers.
Often recruiters are 'try before you buy': you work through one until the employer is satisfied you'll work out, then they move you to direct employment. Usually ads say as much, 'temp to hire' is common.
Some of the resume submission websites of major employers in the computer industry are a mess. If you're running into that the companies probably use recruiters and would rather use their software talent on more pressing concerns.
Some people may find the following statement controversial, and it isn't always true: Big companies will hire from big recruiters. Therefore, if you want to work for a company with thousands of employees go through a recruiter that places thousands of people and has offices all over the country. Medium sized companies will tend to deal with smaller offices and more 'regional' actors. Of course, what looks to you like a 'large' company may be a holding company with subsidiaries all over the country, this would be true for radio and TV properties, for instance. Thus in the corporate sense they are 'medium sized'.
Recruiters To Avoid
If they tell you the position pays a given salary and they take their 'cut' out of that salary and pay you the remainder, then they lied. The appropriate thing to do is demand they pay you what they promised, or leave if they don't. In some states, you could report this to state agencies that arbitrate pay claims.
If they don't know your market (i.e., someone that normally places clerks tries to place you in an IT job) they will do all kinds of goofy stuff. Make sure the company (not just the individual recruiter) is overall focused on the kind of role you intend to fill. I remember in one circumstance telling a placement firm I had just been working on a military base and they said 'We don't place ex-military contractors, they don't know what they're doing'.
If you see a picture of someone who looks like someone born and bred in the US, but the language sounds more like it's from Europe or South Asia, just kill the email or posting. Don't even reply. Don't link to the site. Nothing.
I received an invite on LinkedIn, I told the lady I would meet her in person before I accepted the invite, and we had a pleasant 15 minute conversation. Once I accepted her posting I found out that her profile was updated every time she posted a job on bull.hn, so her profile was showing updates every day, sometimes more than once. I killed that connection in a hurry. It will be awhile before I deal with that company in any capacity.
If staff turnover at the recruiter is significant, it suggests a demoralized recruiting team, usually because the manager is making a mess. This can spill over into your space in a hurry.
In short, don't simply deal with a recruiter as a candidate 'off the street'. You need to build relationships, you need to know these people before you're looking for work, and you need to find people you trust. The good recruiters would repeat that advice verbatim.