As I understand it, all these companies are not really "hunting" jobs for you. They just wait until an airline comes knocking on their door (which they usually do when they have a sudden and/or short term requirement for qualified people).
Since the airlines get in contact with many of these companies when the need arises, they usually pick the cheapest. This means that if you've sent your resume all over the world, when one of them will really offer you a job, it will probably be with the lowest salary.
You can usually trust the majors like PARC, AEPS, etc. but never forget that there are also a lot of crooks in this business ! And don't think that because one of those companies is looking for pilots (on whatever type of aircraft) means that there's actually a job. They sometimes put an ad just to re-fill their "pilot reservoir".
To conclude, this business is like everything else on this planet: you've got to be careful !